Sorry to hear this. Your in a real sticky situation as its your son, but how can he attack you 3 times. Then same again with his grandparents.
It sounds like hes had everything and has had many lifes as a cat. But to be honest weed dont make u do that., i had it for 5 years, but that was in my teenage years and had a temper thats all.
Sounds like if dealers are involved, its got to be cocaine. Id try and find him soon as you can. Sit him down. Tell him he needs get some anger management. To stop the drugs and maybe you will let him come back. Just an idea lol its up 2 you what you want at the end of the day. But if its coke, and im 2 month clean today. He needs get off that shit. But i could be wrong