I have nothing solid to offer. I am dismayed that there has not been a professional willing to begin CBT with an 8-year old. You've probably consulted the gifted professionals on Hoagie's list?

It sounds like he has friends (outside of school), and that the school is a good "fit" in many ways. You've probably been through lists of known triggers for changes in behavior, such as:
- changes at school?
- academic difficulty (ranging form bored, to lack of support, even a sense of unwanted competition)?
- social problems in learning environment?
- friend move away, or ill and out of school?
- new teacher?
- new seat in the classroom? Sometimes a well-behaved child is moved to break up chronic misbehavers.
- any possible exposure to sexual conduct or contact?
- illness/death of a grandparent or other member of support system?
- changes at home such as new sibling, separation/divorce, parental job change, tight budget?
- changes in what foods are offered and eaten?
- HALT (Hungry, Angry, Lonely, Tired), as mentioned in this SENG article?

Wishing you all the best.