Get a keyed lock, I have one on mine have long before SO came along as when I'd go on vacation I'd lock up good even with house sitters I trusted. I have locked it up when SO was accusing my son of stealing change and then pointed out the door had been locked and me with the only key, so he was just full of it and stirring the pot or taking the change himself and forgetting?
That's my solution lock it up

Get a gun.

Well maybe a squirt gun or squirt bottle.

Since your SO is NOT stepping up here you need to take action. This is a 16 year old. A teen old enough to be tried as an adult in a court of law. Go ballistic on him.

There is NO reason for this kid to be in your room. You may not always be dressed. That is creepy and disgusting at the same time. D

It really sucks that your SO doesn't think his kid should respect your space. Everyone needs their own personal place to retreat and your bedroom should be yours. I'm sure you would never barge into your stepson's room or lounge on his bed or hang in his doorway, and it has nothing to do with being in his underwear or not. That's his space and you respect it, he needs to do the same.

The first time my SKs came to our new apartment when DH (BF at the time) and I moved in together, they chased my cat to our room and under the bed. And they just ran right in after him. I nipped that shit right in the bud and told DH that our room was off limits. They could go anywhere else in the apartment, but our room was for adults only. Luckily he understood my need to have a kid free zone. And it's not a SK thing either, even our bio kids don't get get free reign of my bedroom. As a kid I remember my own parents' room being off limits to us kids as well. Adults deserve privacy and their own space. Maybe if you put it to your SO that way, and less like a SK issue, he'll understand. As long as there is plenty of other space in the house, there's just no reason kids should be in an adult's bedroom.

put a lock on the door

I have told dh on several occassions that if he cannot parent his kids then I will parent them according to how I parent my own kids. I let him know what needs to get done and then if it doesn't get done I step in. I have only stepped in a couple of times with ss16 and both times he threw a huge temper tantrum and broke things and once even threatened to hurt me. I stood my ground and told him to leave. When he settled down later dh and I talked to him about how this behavior toward me was not appropriate and how when asked to do something by any adult, a teacher, dad, stepmom, employer, he needs to listen and not over-react. I agree with what echo said, speak up in your own home and if dh and ss have a problem then they can leave, not you. It's your home, take control. Tell ss that he is now old enough to stay out of your room and that if he wants to talk to his dad he needs to ask his dad to come to his room or the livingroom. Also let your dh know that if ss wants to visit with him he should go to the livingroom or garage or outside, where ever guys usually hang out. My ss16 used to come into our room too when I first met dh b/c dh had let him sleep with him when he had nightmares, back when he was 11 and younger, so it took awhile to break him of that habit but I put my foot down. I was very vocal about it. I told dh if he wanted to hang out and watch tv with ss he had to go out into the livingroom or ss's room. It took about a month for them to change their habits. (I also disconnected the tv in my room for awhile!!!! Be very vocal and when ss walks in yell at him and tell him this is your room, start making it just absolutely TERRIBLE when he walks in the door like he is committing the biggest no no, jump up and yell and say don't come in here anymore and don't stop yelling til he leaves and if he hovers open your door and tell him loudly to go do something constructive like dishes. Make his presence in your room feel uncomfortable by get extremely vocal. I don;t know just suggestions!! This is a toughy.

There's NO good reason for any kid to be in their parent's bedroom. Period. Dot. That's the only room in the house that should be reserved for you and your husband and you should be able to come and go without fear or worry that you have to LOCK the damn door behind you.

Serious consequences for his actions need to be put in place NOW, by your husband. He needs to tell him "Sorry kid, our bedroom is OURS, you have the rest of the house, stay out or your phone (laptop, video games, car, whatever) will be taken away for a week every time you come into our room uninvited".

What's so hard about that?

I think your SO needs to spend time with his son OUT of your bedroom. Is he spending an extraordinary amount of time in your bedroom? If so, he needs to spend some time with his son in the family room together, they can talk there. Then when your SO goes to your room perhaps step son won't have to follow and hover because he did spend some time with his dad. this plus your discussion with SS about your privacy might help.

I think your SO needs to spend time with his son OUT of your bedroom.
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^^^THIS! And just read it over and over again. Sounds pretty gross though eh? LOL

This one is simple. Start leaving things your SO wouldn't want SS to see sitting out in the open in your room randomly. I would start with thing that buzz... I had this same issue. All I had to do was threaten to leave stuff out and it stopped that argument right away. Now, the kids wouldn't even consider coming in. Also, perhaps your SO could spend more time out of the room when he is home.