And other reasons a math-gifted student might obtain low grades in math include mind-numbing boredom. Drastically inappropriate instructional level can be very challenging for young children to manage, or even to identify accurately. Some develop perceptions of themselves as being "bad at" a subject because it's so far beneath their instructional level that they struggle to find it intellectually or attentionally engaging (my own story, early on, before an intervention by my parents). Others take it a step further, and perceive -themselves- as being bad.
I would likewise be conservative about the ODD (I'd view it as provisional only), as, between extreme instructional mismatch and weaknesses in self-regulation that accompany ADHD, what's currently perceived by adults as oppositional may resolve at least in part if his intellectual needs begin to be met. He would not be the first HG+ primary-age student to present with challenging behaviors principally because of an inappropriate instructional environment.
