Sounds a lot like my DS. He's very happy playing lego and reading all day.
I've also found it really hard especially when there are really lovely kids who want to play with him but he doesn't feel like it and doesn't seem to realise or be particularly bothered that they may stop asking to play if he says no all the time. It's certainly a tricky thing to balance encouraging the development of social skills and friendships without pushing his poor little introvert self too far out of his comfort zone.