I love the advice others have offered (extra one on one time, asking him what's making him angry, more structure, exercise, take away extras). My heart goes out to you--time will help. One parent on another thread suggested monitoring diet & sleep.
My most difficult DS responds to one on one time, exercise, alone time (when he's misbehaving, sending him to be alone--without technology around--essentially either to lie down or read or just think/daydream..kind of like a "time out" but not labelled as such). Does your DS have his own room? I've told my son that if he behaves inappropriately, he needs to do it in his own space, not in public. He generally reappears in a little while a completely different person.
If he's underchallenged at school, steadily work to address those needs, but panicking and reacting to bad behavior (we've all done it--or at least I have) isn't as effective as reason and patience and perseverance. You may not find the right fit academically for him since he's very advanced, so you might have to supplement at home. ALEKS for math, educational videos like Planet Earth or Bill Nye, keep reading good books and discuss if you can, puzzles, board games.
My difficult DS's perfect behavior and angelic disposition appear when I devote myself to him entirely. We'll bake together, play board games, practice the piano, work on his homework, do a puzzle, etc. Of course I don't have that kind of time all the time. But if I do that every once in a while, it seems to bring the old DS back.
Lastly, depending on many other factors that I can't evaluate, you might consider ADD or ADHD and medications. It can make the world of difference to a family.
