I don't know to what degree you're involved in your stepkids lives but ultimately they're your stepkids. Even if you would do differently (and better) if you were their parent, the ultimate parenting decisions belong to their parents. You can maybe talk to your DH about what you're willing to put up with or not put up with that impacts you directly (set personal boundaries) but you can't force them to be better parents to their kids if that's not what they want to do. It sucks to see SS affected by his BM's questionable lifes choices but it's not like you can make her choose better or do better by him. It'll feel like the world's going crazy all around you and why won't people just pull their heads out of their asses and see the light already?! 

My SKs are either in early adulthood or are rapidly approaching it. For several years now, I've expressed to my DH what I expect. I will not live with adult SKs long-term. Period. End of discussion. They can have six months post-college to get their ducks in a row if they need it, and I'm fine being an emergency landing place if something happens. DH and I were both "boomerang kids" after our respective divorces so I don't want to be a hypocrite that says they can never come back. However, both my SSis and DH's brother have re-nested for likely the long haul, and I don't want to live like that.

Now, I say that because I have two neurotypical SKs. If one of them had autism, I'd have to either shift my attitude or admit the relationship I'm in is incompatible. Your OSS is going to need a lot more guidance than BM and your SO are providing, and depending on his own drive to want to fly from the nest, he may taake a few more years to fully launch. But again - he'll only ever even have a chance of launching if your SO gets his head out of the sand.

What YOU can do is figure out your boundaries and expectations for the kids as they age, and it's perfectly fine to have different expectations based on their respective capabilities. Will you be okay if they live with you, and if they do, what type of behavior and contribution do you expect? How long do you think you can allow them to live with you? If you can't live with them, can you keep a relationship with SO but live in separate homes? If you and SO have joint finances, what are you comfortable with him spending on his kids from any mutual funds? What are your mutual financial or future goals, and what do you expect his contribution to be?

Once you have a solid understanding of what you want and expect, lay it out to your SO. Schedule a time to discuss this where he has time to think about it, too, and when life isn't anymore stressful than usual. It should be a time when you two are alone and have ample time to discuss, take breaks, and regroup. I recommend food but no alcohol during these discussions. I'd also recommend that you have a safe phrase that can be used to temporarily end the conversation, but ending the conversation needs to be met with a new date and time to discuss. If it feels like the conversation is circling or you're both having problems communicating, be willing to see a couples therapist to help you talk through it and/or write letters to one another expressing your viewpoints. Sometimes how we say things versus what we mean don't communicate the same.

Your SO can't make BM do diddly squat. The only person he controls is himself. OSS will be 18 soon, which means OSS will have a world of choices open up to him. While your SO may have felt his hands were tied before, he'll have a lot more breathing room once OSS can't be controlled by BM unless he wants to be. Your SO needs to be prepared that BM will kick out OSS because he's an "adult", or that she'll continue to treat him like a kid, or she'll turn extra toxic and expect him to behave like an adult while keeping him locked in kid status. The fact that OSS says he feels stuck should be the boot in your SO's behind to push him to act for his SON, not out of fear of BM. Bring that up to him and see how he responds.

Finally (because this is long enough), we had a similar situation where my OSS disappeared one night and no one knew where he was. My OSS doesn't have autism, but it was still very out of character for him to not come home and not answer his phone. I fully supported my DH going out to look for him (while calling and texting OSS to tell him he was in deep sh*t if he didn't respond soon). DH and BM coordinated over the phone, but he wasn't about to pick her up and drive her around. He'd have to listen to her lose her mind and probably blame him for it all, and he just wasn't going to deal with it. OSS eventually came home (I think he had mentioned to BM he was visiting a friend but they switched locations and stayed longer than anticipated or something) and DH met him at BM's house to let him know what he did wasn't cool and worried them all. Then he came back home to me and probably smoked through another half pack of cigarettes to calm his nerves.

I think your SO likely reacted like a normal parent when their kid does something out of character. If OSS did this every week or something, then his reaction may have been too much. Expecting 17 year olds to make rational decisions all the time is asking too much, especially when you have a kid who is grappling with issues that they don't have the mental capability to address on their own. My guess is that it will happen again, so your SO should focus on how to panic less in the future and teach OSS methods for dealing with his emotions. Taking a walk is perfectly fine, but maybe the expectation is that he leaves a note or takes his phone and turns on GPS or sends a text saying "out walking", etc. Whatever it is that OSS can build into his routine to do that makes sense to him should be what SO works on with him.

Yikes what a mess. If you're worried about the boundaries on BM I would continue to monitor the situation to make sure your SO is continuing to focus on OSS and how to help him and be on the lookout for BM to try to weasel her way back into your SOs life. Just be aware and point it out to your SO if it starts to happen. Don't let BM make it about her when the issue is what OSS is going through.