The US Department of Justice says that it's illegal to keep kids out of gifted services on the grounds of disability.

https://www.justice.gov/opa/pr/larg...-lawsuit-alleging-discrimination-against

You can also look to the Office of Civil Rights for help: this is Wrightslaw's page about this situation:

http://www.wrightslaw.com/blog/school-says-no-advanced-classes-for-kids-with-ieps/

The key word you want to use in this conversation is FAPE (Free, Appropriate Public Education). Becuase your child has a disability, he is legally entitled to FAPE. This means that he should be given an education appropriate to his needs, which certainly includes gifted services if he has qualified for them.