The trouble is you are looking at this from a neurotypical point of view. Neurotypical people grow up acquiring social skills naturally and understanding people without having to think about it, and as a consequence, know (usually) how to behave in any given social situation, which includes making friends and maintaining friendships.
For those with autism the world is a confusing place, as they do not acquire these skills automatically. Trying to navigate the plethora of unwritten social rules and niceties is hugely confusing, immensely stressful and often scary.
Autistic people don't have the same need for friendship and social interaction that those of us who are neurotypical do. They are wired completely differently and are happy spending time following their regular routines, engaging in their special interests and hobbies on their own.
School (and especially a mainstream one) will already be stressful enough for your son, being surrounded by people all day, so he needs his downtime at home on his own to relax and unwind, and feel secure.
As he gets older he may change, but for the moment I would listen to him and leave him be.