The ways of making and keeping friends that come naturally to everyone else don't come naturally to him. He *wants* friends, but just doesn't know how to make them.
They don't come naturally. Those are skills you have to learn. Most end up being naturally taught, but some people miss it.
Without proper supports*, he quickly winds up socially isolating himself with his actions
Which he will always do until you teach him better social skills. Don't look at it as some medical disorder, look at it as a skill gap. I have the same problems and I'm 28. There's no one to help me now and no one ever tried. The older you get with poor social skills the worse it gets. People don't fault kids for social awkwardness, but you're a creepy crazy guy if you're socially awkward after being a teenager. People don't give you any benefit of doubt. You exist to kidnap or harass people as any mistake you made was on purpose and for some specific reason. 'Why does this person not have any friends? He must be dangerous, lets stay away from him.' Social awkwardness is a self perpetuating problem that can quickly spiral out of control into deep depression and suicide.
You won't help him by teaching others how to behave around him. You can't teach everyone he'll meet in his life. You have to teach him how to interact with others.
Worse, he will interpret any sign of kindness as friendship and follow this person.
Because everyone else ignores (or hates) him or at least that's how he sees it. When no one talks to you, that one person who does must be really interested in you. Why else would they talk to you? No one else does.
Please don't let him end up like me. Teach him social skills like you might teach someone math. Read about some specific type of communication or behavior and then practice and practice and practice it. You might have to deal with anxiety issues as well.
Please don't assume that a) your experience is universal (anecdotes are not data) and b) that those of us with kids on the spectrum are lazy, ignorant twits. We have spent YEARS doing all of the stuff that you "suggest" and all the stuff that you deride as "useless" and guess what? Our kid STILL has a real set of problems. Is he much better at social interaction now that we have spent all this time teaching him skills and getting him therapy? Yes! Can he pass as neurotypical? Not for more than about 30 minutes on a good day. And that is never going to change, no matter how much willpower, time and money we throw at him because there is a real physical problem here.
Would you tell a blind kid that they just need to work on the skill of seeing? Of course not! And ASD kids have real, measurable, neurological deficits. The brain is a wonderful thing and can route around a lot of damage, but it is no substitute for dedicated hardware, which he clearly lacks. And quite frankly getting guilt from pseudo-experts like you does not make life easier. My wife has a masters in this field, which required actual lab work and clinical experience, so please keep your ignorant, holier-than-thou, superior attitude to yourself. You are causing a lot more damage than you realise.