Self Confidence

I don't know if this is behavioral or not, but my son has some issues with self confidence. Even the teachers say he doesn't join in group work right away and he doesn't take risks.  He lacks self confidence. In his gade level at school, he won "mr. math" and even as early as kindergarten, he got an award for being "time keeper." He has always been all about numbers and has always gotten really good scores in math in class, on testing when he was evaluated and even on state required tests. All above average. That is his strength academically.  But, he lacks this confidence. He doesn't think he can do it. I always tell him he does really well in math, there's no problem. At the end of the year he said it was getting really hard, but he got an A on his last test (really on all his tests) and on his report card. I worry because he is the perfect candidate for the accelerrated math class this coming year and he is worried about it being too hard. It isn't only with academics. It's with a lot of things, math is just what jumps out at me right now.

How can I help raise his self confidence? Can the schools do anything to help with this?

Btw, he is 11, has Aspergers and ADD and going in middle school this Fall.