“My question is how to enforce he adhere to guidelines?”
Depends on the guidelines. The only thing you can really try to enforce is how he behaves when he’s inside the house, so that depends on what he already does inside your house and what you don’t want him doing inside the house.
About your neighbours and the community…trust me when I say that worrying about stigma will only make your life harder, not easier. That’s not to say that you have to go around handing out fliers saying “Hey! look! my kid is OTD!!!” but don’t bother trying to keep secrets. It will come out anyway, and when it does you’ll be embarrassed for hiding it in the first place, and it just gets in the way of you doing what you need to do for your family. Also when a kid knows that you’re ashamed of his image in front of others it’s very degrading and kills self esteem.
Back on point. How to enforce it? Your only real threat is to kick him out and that shouldn’t even be on the table unless he does something exceptionally unacceptable. When you talk to him and lay down guidelines, speak gently but firmly, and from your heart. Respond to his anger as calmly as you can, and tell him how you feel about it. You don’t have to be a wimp about it, but expressing your feelings will disarm him in the long run, and send the message that you really do love him despite this rough bit. Tell him why you need him to keep these guidelines, bring up the siblings, do not bring up the neighbours, or shadchannim, or any kind of reference to public image.
If you can, I would recommend having an honest and frank conversation about all of this with him. Don’t talk down to him, or treat him like a kid. If he wants independence and adulthood then treat him as such. Honest conversations never hurt. Even if he yells at you again, he will eventually remember it all when he feels like coming back. Lay it all out for him. Explain that adulthood comes with responsibilities, among which includes acting responsibly around his younger siblings, etc.
Just one more thing. The fact that you’re asking for advice here, and judging by your posts here, it is evident that you love and care for your son and his wellbeing very much. I would have been honoured to have a parent like you, and I wish you hatzlacha with your son. Many of the parents I’ve dealt with aren’t half as caring as you appear to be.
I keep thinking of more things to say, I’ll wait for your response before I say any of it.