I am an Intervention Specialist and there are a few things that you can do. First, in order to prove the need for a 504 the student needs to be performing at least 2 standard deviations from their norm. In my brothers case, he took the ACT and got a 32 first try as a sophomore, but was receiving C’s, D’s and F’s in his classes. In order to prove that he needed help, my parents pointed out his test scoring in comparison to his assignment grades. We found that at the high school level some teachers were not willing to accommodate him by signing his agenda to verify assignments had been written down/completed either.
IQ is an indicator that can be used to determine if a student needs/qualifies for services but is not the only or most reliable indicator.
Every school district has parent advocates available for your use. I recommend getting a hold of an advocate and seeing what they can do to help.
The official term for a student who has a disability and is gifted is “Double exceptionality”. This is just being recognized in the field of special education. Some school districts and teachers are informed of it and others are not. I don’t know if your son has been officially tested through the district, but every parent has the right to say I want my student tested to see if they qualify for special education services and the school must test him. This does not mean that he will qualify for services but official testing could provide evidence to support your position that your student is both gifted and has a disability. Frequently this testing will show gaps in his executive functioning’s that can be significant enough to qualify for a 504 if not an IEP. If he is officially in the gifted classes then there is some testing on file to determine that he is gifted and you can use some of that information to help prove the two standard deviation of lack of performance.
I hope this helps! Good luck!