I also only have 3-4 children. Yours sounds like mine but mines not as quiet. He will laugh and engage with us, just not as often as others. He will be playing nicely but when mom or dad comes he becomes quiet and non talkative...it's a bit odd lol but I've never considered it to be an issue to be honest.

I'm not sure what you can do other than invite him to play with the group. It will be up to him if he wants to or not. This might just be his personality?