What is the plan for the 27 year old?  Has he been evaluated for other issues aside from epilepsy?  (my YSD has it.. is able to work and has no impact on her life except for daily meds.. but I realize not everyone is in the same place)

Why doesn't his mother want him to be healthy?  It never should have fallen to you to try to parent him on this topic.. this was a HER problem.. 

It seems like she is the problem in her permissive parenting.  You have spent 5 years here.. do you see a future if her kids never launch?

Yelling certainly was not the most calm course of action, but fully retreating is an naive move IMHO.

Be clear. He either showers, wears his seizure alert 24/7, and no longer pollutes your home with his odoriferous emanations and his break dancing episodes because he won't manage his condition, or he is out.  Start with kicking him our for the usual hours of the work day, give him no access to the home, and he only returns when either you or his mom is present.  If you come home and he reeks, out he goes. He can occupy a lawn chair in the back yard. Toss him a bottle of Dawn dish washing soap and point out the garden hose and he can bathe himself and his stanky clothing only regaining access to the home when he and his clothes are odor free and dry.   Hey ,if Dawn is safe for ducklings and other oil soaked wild life, it is safe for reeking Kidults.

For some reason your SO thinks that enforcing standards on resident Kidults is a problem.  Nope, far from it. Standards of behavior and Standards of performance are even more critical when there are resident Kidults in the home than if there are minor children in the home.  Kidults are resident at the pleasure of both SOs. If one vetoes the presence and residence of the the Kidult, the Kidult is gone. Period. Dot.

Make sure your SO has this message clearly presented. to her and her resident Kidult spawn.  Her stench infusing spawn is either clean or gone. It is the Spawn's choice.   Enforce the standards and make sure that SO knows that if she is too lazy to enforce the standards, it, that you will and neither of them will like it.  Body odor, nasty oral hygiene, or rancid breath and ... he is in the back yard to correct the condition before he returns.

In addition to mandatory daily bathing and laundering of clothing, invoke a daily red die dental tablet test for his nasty teeth. Any red or pink, he goes and bruses until all hint of red is gone.  

Certainly you can sit them both down and express your regrets for losing your cool. However, that expressio of regret has to be 'followed with firm dictates that the stench ends or SS-27's presence in your marital home ends. This is not a child. This is an adult.  

Lather, rinse, repeat.

While I have not had a family member struggling with epilepsy, I have a very close friend who worked for me for years who does.  She referred to her seizures as "break dancing" episodes and was very diligent about managing her meds and keeping her rest levels regulated. When she blew her work and rest schedules, or over indulged in libations, within a day or two she would have a seizure. 

Stay calm, be confident, be assertive.

Stop being tolerant and tender. Start being effective.  He pulls his head out of his ass or he lives a life of constant pressure and humiliation.  Pain is the ultimate teaching tool.

All IMHO of course.

He's 27 and can handle be treated as an adult.  In fact this coddling being encouraged, this putting it all on his BM has gotten you to this point.  IMO he should be treated as a roommate, who should be chipping in financially and with household responsibilities.  It's no wonder you lost it.  
 

Expectations are not a bad word and you need to put some on both of them.  Once a skid becomes an adult,  that should change. He has a condition that can be managed.  He's not doing that.  No more excuses from either of them.  This is unfair to you.   

On this?  You spent 5 years and are nowhere.   BUG RED FLAG.  BF your SO is doing nothing.  He can't even get him to shower.  The basics. He is accepting a smelly kid [ adult ]  knowing that this doesn't go well on job interviews, or life in general.

Time for the old talk.  '''This is my life''' if you want a relationship with me certain thing must change.  SD must shower ever other day [ giving something] change his clothes. To clean ones.  
TWO   SS must get a job in two months and make move out plans.  You are not his mother. And can't live with two adult men.in the same house.  You need a '''TIME LINE'' on move out date. 
 

Unfortunately, things like this '''disney daddddy' don't change. You must start making a exit plan. Getting money in a bank accounts in your name only. Asking about apartments. Ect. 

There’s gotta be a reason why she—let’s call her Mommy-B (B as in ‘bitch,’ and I don’t mean the canine variety)—has been your “fiancée” on layaway for not one, not two, but five damn years.

So, let me get this straight.

You’re supposed to share living space with a full-grown, slovenly man-child, marinating in his own body odor like some dead corpse, and you’re NOT allowed to lose your shit? Meanwhile, you’re the one keeping the lights on, paying the bills, and what’s His Royal Slatternliness contributing? Beside him crapping in the toilet, daily?

And instead of taking a stand, you rolled over, showed your belly, or else  Mommy-B was going to smack you with the newspaper?

Damn. She must be a damn GODDESS in the sack, because you’re out here bowing like she’s the Queen of England.

But let’s cut through the bullshit.

Your problem isn’t with Mr. Slatternly—hell, he doesn’t care as long as you keep your mouth shut AND you stay out of his way.

Your real problem is Big Mommy-B. Because she’s the one propping him up, defending him, enabling him, and making you out to be the big, bad stepdad for daring to expect some basic human hygiene.

Face it—you’re not fighting him.

You’re fighting her.

And brother, you’re losing.

So… what’s the plan? Stan?

Well, that depends.

Depends on whether you own the place or if the lease is in your name.
Depends on whether you can separate your finances before they drain you like a busted gas tank.
Depends on whether you can move out or, better yet, get them to move out.

Because let’s be real—you’ve spent five years at this crooked gambling table, doubling down on a bad hand.
Maybe it’s time to admit that in five years, nothing has changed.

And why?

Because she doesn’t want it to.
That’s why.

So now, you must change.
Not for her.
Not for Mr. Slatternly.
Not for “family harmony.”
But for YOU.

Why?

Because, if you haven’t figured it out yet—
You are alone in your relationship with these people.