OK, I didn't see IEP mentioned anywhere, so I'd consider asking for an IEP eligibility assessment, unless you're satisfied with an update to accommodations on your ds' 504. The primary reason I'd suggest an IEP eligibility evaluation is that I think it would be helpful in understanding your ds' behavior in class to have an SLP eval as well as possibly the cognitive vs achievement testing.
Your school is functioning a bit differently than the schools my kids have been in, so I don't know if you might be able to get an SLP's input without the IEP eligibility process - our schools would never have provided an OT eval without going through the IEP process, so it sounds like you have a school that is a bit more reasonable in working to meet the needs of students, which is a good thing!
Why an SLP assessment? I just wonder if your ds perhaps doesn't have some challenges with either receptive or expressive language. They can both be extremely subtle but also extremely frustrating for children and they also can result in what looks like behavioral challenges. Anyway, I could be way off base and I'm not a professional I'm just a parent - but it's one thing I'd wonder about.
Hang in there and let us know how everything is going!
Best wishes,
polarbear
