If he were my brother I'd tell him that firstly, not everybody finds someone to romantically love in life and the ones who never marry/have kids etc still live happy fulfilling lives. Marriage/children can add a lot to one's life but aren't necessary for happiness!
Him wanting kids now is not great. The older the father the more risk for congenital defects. This is hard but he needs to hear it and decide how important the notion of biological children is and what to expect if he does go that route (assuming he meets a nice woman who wants similar)
He needs to be able to live happily single before being with someone. That's Relationship 101, don't get involved before you're on solid ground yourself. It's a recipe for disaster to expect your other half to make you happy! So he should be working hard on his own happiness and life satisfaction now before he (hopefully) meets a nice woman.
I don't mean any of the above in a rude manner. He can be told lovingly all these things so his expectations are reasonable.
I'd strongly suggest he joins clubs, groups, any kind of thing away from home with other adults to interact with. If he doesn't enjoy a group/outing, he doesn't have to return but he should try to get out twice a month to start with. Extra bonus points if the group/outing includes others on the spectrum, he may be pleasantly surprised to find he gets along easier with another person on the spectrum and doesn't feel as self critical.
I wonder if he's fixating on this subject in response to his depression and lack of job/social life? If so no advice for him other than to work on that and not sit thinking the world is passing him by and feeling lonely. It's a slow process but he could really improve his life if he was honest with himself. I hope he finds some nice hobbies, friends and maybe a romantic partner. 💘