all i can do is give you lots of and let you know you are not alone.
my eldest son is 5 and seccond son is 3, the younger one, Trent is always bashing his big brother, seth has scars on his forhead, one on his lip from were he was body slamed into the floor. they havent faught for a while now, so there is hope that he will grow out of it. like you, we have tried smacks, seperating and time out, if it dose work, it will only work for a day or 2.
i found that i just had to kind of stick it out and discourage it as much as possible and get either 2 of everything, or make the more violent one have first choice. good luck, and rthere is hope he will grow out of it, untill then, big squishy s