How long has he had his diagnosis of autism? Was he diagnosed as a child or as an adult? Has he had anyone to talk it all through with? Any support or been taught any skills that he struggles with?
See, my children both have autism. Severe enough that they will never marry. Never have a job. Never have a child of their own. Never live independently.
They are very loving. We have also spent a lot of time teaching them about social interactions and continue to do so. There is nothing about autism that means you don't care about people. Maybe that you need to be taught how to express it, taught the social niceties that NT people (generally
) just seem to pick up.
If your brother has a diagnosis of autism, this is not a reason to shrug and say oh well, he's not really responsible. If he can marry, have a child and hold down a job, he can be taught that you keep in contact with people. He can be told that people care about that and because there is nothing about autism that means you don't give a shit, if he knows that it matters to others, and he loves and cares about them - he can do things that make no earthly sense to him but which he understands matter to them. iyswim.
He has a wife, he has a child, he is obviously capable of loving and being active in someone's life.
I think you need to accept that maybe, just maybe, he actually feels, with cause, that he doesn't want much of a relationship.