DS will be two in three weeks. In the last two months he has become quite aggressive. When he's told no he will literally run and find something to bang his head on and if he can't find anything he'll use the floor.
He smacks me and DH when he's annoyed about something and throws himself backwards constantly.
It came to a head this morning when he fully fist clench punched DD (4) in the face 
He's a perfectly healthy boy in every other aspect but he's non stop on the go with no real concept of personal safety.
The aggression is definitely not learned behaviour but it's awful and he's hurting himself. I don't know what to do.