Social development is such a complex subject. Although its a skill that can be learnt it is also an ability that might not be very well developed within a child. Just as a child can have problems with eyesight or hearing some can have problems with social skills. Forcing them towards social interaction when they dont feel ready or when you dont fully understand why they are reluctant could be asking for trouble. Be lead by the activities he enjoys doing at home and see if you can encourage him to take it further but in a less organised way. He could just be shy and I'm not sure thats something we ever loose, as adults we learn to compensate but when you are young being forced to mix with others when you dont feel up to it is pretty challenging. Its a bit like being made to join an elite swimming club when all you want to do is float up and down in the shallow end. Have a chat with his teacher its amazing how different some children behave in school to out of it. On the other hand he might perhaps be having a problem at school that he has not told you about, that would be another reason why he is happier at home.
What gives you a warm feeling? 🥰
If I had to choose.....just one day