If the math consists of a lot of rote exercise, then that's not a surprising outcome. In fact if he's that good at math he may know exactly how many problems to skip and still pass. Once a gifted person masters something, they have little patience for repetition. So it might be they just won't do the work, or they intentionally do it wrong. The gifted also have a quirky sense of humor, so there could even be some sort of joke embedded in the rebellion.
I would make sure that the ODD is not a misdiagnosis given the IQ. Gifted people can be pretty unruly, especially if not in the right environment. As an adult I keep going through periods where I think, maybe I'm not gifted, maybe I have ADHD, or ASD, or... lots of things. What I've learned is that the symptoms of giftedness overlap a lot with more common diagnoses.
I'm speaking from a place not of professional knowledge, but personal knowledge. I'm gifted+LD. At least that's what the paperwork says. Other people will be along who have great professional knowledge.
Of course, me being me, I like to get answers from a book. Most of my reading has been focused on the adult experience lately, but these might be helpful. I haven't read them personally, but they're on my to read list possibly--my daughter isn't quite 2.
Misdiagnosis and Dual Diagnoses of Gifted Children and Adults Emotional Intensity in Gifted Students: Helping Kids Cope with Explosive Feelings You're not alone.
