While I have a 2e child, I do not have a child on the spectrum so my experience here is limited.  I am wondering, though, what the requirements for admission to the GT math program are.  Is it based solely on NWEA MAPs scores or is there a second measure such as a group ability test or something else and did he meet the requirements across the board?  If he meets all requirements and they are denying him access to the program solely because of his disability, I'd think that you'd have a good case.
Also, what have you tried thus far in terms of trying to get them to reconsider?