I don't think you're a bad person at all! I think you're being a good mother by putting your son first. It must have been very difficult to make the decision to live apart from your SO, but you did it for the good of your son and no one can fault you for that. You aren't selfish. Your SO's son is not your responsibility but your son is, and if you felt them being in the same house was detrimental, then good for you for getting your son (and yourself) out of a bad situation I don't blame you one bit. So don't sit there and beat yourself up. You tried and it didn't work out. Sometimes it doesn't. Better that you found that out now instead of after you married him.  I've never dealt with ADHD but I can imagine how difficult it is and there's no need to be a martyr for someone else's son. Hopefully his parents will learn better ways to deal with his issues or he may never be an independent person. 

PS Totally laughing at "I want my son to be an astronaut, not a chicken nugget."

That's not all because of adhd. That's a 13 year old who has been allowed to behave that way. Adhd can be managed.

He needs to parent his child. Kids with adhd need more structure and parents who don't just roll over when they throw a fit. Even adhd kids who learn they don't get their way by behaving badly knock it off and make different choices. 

The noises, the inattention is probably adhd. But the tantrums and fits may be more related to "why shouldn't I do what works?" 

Doesn’t have to see what he doesn’t want to see.  Or do anything about it.  Why differents will a year make if SO doesn’t do anything ?  Maybe you too will put your head in the sand ?  You have to do best for your son. Have to do the right thing.

 It’s hard enough when SK are somewhat normal, but to deal with ADHD, and it’s sounds it’s more than that.  This kid should be tested.   It’s really going to strain your relationship,  and not fair to your son. 

For one thing - the chicken nugget song sounds fun. That's not a big deal. So really examine your own feelings and learn not to sweat the small stuff. We have boys 10-12 together and we sing Diggy Diggy Hole all the time. My partner is an academic scientist and I also work in an intellectual field. We have high aspirations for our kids and don't worry about them becoming a cartoon dwarves. And yes, all of the kids in our family are weird. Weirdy-weird. They'll be fine. 

However, it's not just small stuff that you're describing. There are serious mismatches in discipline and parenting approaches and your partner has his head in the sand over issues like limiting time on devices, adequate sleep and hygiene. Couples counselling defo. He is seriously messing up his kid's chances - this kid needs more routine, more discipline and more help in learning how to self-regulate. As an ADD kid raised by an ADD mom - well, I know how hard it is when your own head is all over the place and you don't have consistent parenting. 

My exH was a good man and decent enough husband, but a terrible parent (is a mild descriptor).
It is not fair to describe someone like this as amazing, unless you mean that you are amazed at his sloppy, inability to raise a child properly. Then it is not amazing but a tragedy.
We should not get to seperate out the parts we like about our partners, yet blame or resent their children for what is obvious failings on the parenting part of their lives. Some times that bad behaviour we see in our minor children / step children is a direct reflection of the job (or lack there of)  we do as parents.

My situation was similar to yours in some respects. I was very concerned about the horrid influence my stepson had on my biological son. (They are now 18 and 13 respectively.)  In reading your post, this is not only an ADHD influence - you can probably medicate that to reasonable levels of "normalcy". (Please excuse the lack of tactful language.) Your stepson has learned and patterned behavioural issues - whether he has ADHD or not. The ADHD is an excuse to not deal with the underlying learned behavioural problems - problems for which there have been no consequences or proper parental corrective direction.

You can discuss this with your partner - and set the necessary corrective action. Some things are not negotiable - at all. Both my SS and BS know that teeth and hair are brushed in the morning - at night again for teeth. Shower and hair washing happens at least once a day - they can choose morning or evening.  It is a normal habit for them that comes from expectations that were set years ago.
"You do it because I said so." Very seldom have I taken this stance with my boys, but certain things are just not negotiable - including hygiene or food/snacks in rooms overnight. Doesn't happen. House hygiene rule. End of story. Step out of line and there are consequences. Some parenting aspects are not discussed or negotiated with the kids - they are ordered.

Children need to know what your rules, boundaries and expectations are - and they need to be consistently applied. It is extremely hard when  you and your partner have different expectations and rules for your bio and step children. In my case, it couldn't work. I had to parent my own son, set rules and expecations, I disciplined my son. My stepson? Ran wild, no rules, manipulated Dadddeeee, did as he pleased. My husband couldn't - rather wouldn't - see the damage this was doing. And that is why he is my EX husband.   

You and your partner need to get on same page with the expectations for your children. If  you can not, do not subject your child to the feral behaviour of someone else's failed parenting. Eventually your own child will see you abiding by that poor double standard.

I doubt it is a case of your kid is better or that you are better. I never felt that way either. However, you DO KNOW better. That is the difference. Make it (the difference)  or break it (the relationship).
(I did both... Divorced my husband, kept his child - who now actually is far far better behaved without his Dad's influence than I could ever imagine. Stepson lives  with me under my rules, and I am no indulgent Dadddeeee  with divorce guilt... My stepson is actually a decent young man who is doing well at college - no thanks to Daddy.)

Why wait a year?
Why waste your time trying to accept the unacceptable?
This man is showing you who he is now.
You already see and feel the red flags.

The Xbox and video games are parent to his son.
When you point out issues with his child, he deflects and points out what is wrong with your child rather than address your concerns about his child.
He uses ADHD as the reason when he is a poor parent.
Ugh no.

You sound like a reasonably concerned Mom.
You did the right thing to not have your child influenced by your SS and your SO's low expecations for him.
Move on.
This guy is Mr Right Now, not Mr Right.

My skid has ADHD and most of what you listed is just normal teenager behavior. The inability to focus is usually chalked up to adhd but the hyper focus on video games is also just boys being boys in this world. He’s inability to be social is probably more attributed to his fixation on gaming instead of interacting with friends in the real world. Kids won’t have social cues if they’re never in a situation to practice them with peers. So many parents use adhd/add/odd/etc as a cover for just bad and lazy parenting. I would know, I’m dealing with one (BM). It’s so easy for them to let them sit in front of the tv instead of having to deal with their child. 

The problem is not your skid but his parent. You need to redirect your frustration on to your DH for failing his son and see him for what he’s doing then decide if that is who you want to be with for the rest of your life. He is not likely to change. He is likely to let his kid game his way through his learning years then fake shock when the kid fails to launch and sit in his basement for the next 30 years. You have options: call your dh out on it and see if he will change his parenting or don’t and decide if you’ll stay or go. You can’t expect a kid without proper tools (parents who care) to develop like yours. Stop comparing your kid to his and stop letting people insinuate adhd is some type of crippling disease. It’s basically just his brain is moving to fast for him to keep up...which is why visual graphics in virtual reality holds their attention because it’s extremely fast paced. The kid would be fine if he had a bio that parented him. 

My skid is 14 and has ADHD as well, plus other mental disorders. I also have a 3 year old with my husband and another on the way. SS14 is a nightmare to deal with and DH enables him. My DH also compares SS with DS which is crazy, since DS is 11 years younger than him. Plus, DS actually has more household chores than SS. DS will push in his chair and clean up after himself when he makes a mess at dinner - that's way more than SS does. It's sad and completely DH's fault.

I don't think your situation is going to get better. It will only get worse, especially if you also have an enabling husband, a useless BM, and you are disengaged. Plus, teenage years are the worst! You seemed to have already moved out and you don't have your own children with him, so you don't have to worry about a continued relationship between your child and your stepchild. It might be the best thing to end the relationship. Even if you think he's great now, you'll probably end up resenting him due to his poor parenting.