My daughter is 15 and has been the same way for years. The fact is that many children on the spectrum just simply do not click with most people in their age group so they give up. You mentioned he only wants to talk with adults and that is due to the reason I stated before. They will be ‘shy’ and hesitant to approach any of their peers because of they so strongly believe that there is no point. Its the same when they are approached, you may notice that your child will ‘reject’ people because the simple truth is that they don’t care for most people around them. Having your kid go to social functions isn’t going to help at all because they’ll hold the same mindset either way. Unfortunately, I have only seen my daughter hold friends she met online which can sometimes be dangerous. You may want to check with him that he has online friends and that his online interactions are appropriate. To add, if you’re worried about him talking to himself, sometimes it can be normal. But if he's having a full on conversation where he’s replying like somebody is actually there you may need an intervention. I noticed when my daughter was at her lowest point in mental health, she would just talk to herself in the oddest manner. So theres a line where it becomes more concerning than normal.

For many people, memories from the past are very vivid and it's almost as if they relive moments that happened years ago. While my memory isn't the greatest (thanks, ADHD, lol!), I still remember some things I said and did at say, 10, like they didn't happen all that long ago (I'm 40 and autistic). Many autistics also perseverate on things, and this can include things from several years ago. Perhaps he could join some groups or clubs in school with other kids who share common interests :)

I appreciate the ideas as a matter of fact I have just looked into homeschooling for him this year

hmmm... wonder if theres like a social group in your area where he could make a friend? Or maybe try homeschooling and going to home schooling feild trips in the community where he could meet other kids his age? or maybe try having the school let a kid be like a peer helper/ friend to him that way he could get to know someone his age?
just some suggestions...

This bothers me. Isolation, or the feeling of it, isn't pleasant. I worry about my daughter. She's 6. Doesn't make any friends. Try social groups with his age and condition. My daughter has a cousin whose 5 and neurotypical. You could use that as a gateway. They get on great, and family children can introduce them to their friends for social contact.