Definitely try putting him down for a nap after school, or moving his bedtime earlier. When a kid this age doesn't get enough sleep, it's transformative.
That's the quick fix. There's also the likelihood that he's acting out because his class is a terrible fit. Kindergarten in particular can be a horrifying experience for a gifted kid. It goes far too slow, and it focuses on skills far below the ones the child has already mastered. It's also the first school experience, where a gifted kid comes in excited about the opportunity to learn, and finds they're not learning anything.
There are no quick fixes for that. Good luck.
