Funny to read this because just the other day I thought about how many boys we know that as preschoolers were in trouble for being antsy, but now as teenagers do just fine. The expectations were really just not appropriate with where they were developmentally. The reality is that sitting at circle time for academic content is really not going to be a good fit for a lot of kids - not for those who aren't ready for the content, not for those who are past it, not for those who need to move.
I think it should tell you a lot that in his science class he's fine. If you do follow through with an evaluation I urge you to read the Misdiagnosis book first.
One alternative to the chair might be to sit on a cushion that provides more sensory imput. These are often suggested by occupational therapists.
http://www.therapro.com/Air-Cushions-C4218.aspx Also, some kids are helped by having some kind of "fidget" toy they can hold in their hands.
