I like Fanged Bunny's suggestion.
My boys were never very aggressive in general (we're the more quiet reserved types) but .. when my eldest started kindergarten, he did some shoving and pushing that was unusual for him. He had been in a daycare where it had been very loving since he was a toddler. School had new boys some of who were just typical boys, aggressive at times - and he kind of fed right into it. He liked getting a reaction from the other boys - and he was just trying it out.
We did talks about how rough behavior (shoving, etc.) was not ok. But figuring out WHY (as FB says) is key. Ask your son why he's acting like this. Not in an accusatory way - just listen and let him know you won't be angry - you just want to understand. Then say it back to him. And then involve him in ways to better deal with things that upset him. Using his words, exercise to let it out, etc.
Make sure he's getting enough rest. That was a big one for my son. He was going to before and after school care as well as school and he was exhausted. Any change like that with your son? Maybe move his bedtime up a bit and see if he does better. Talk to his teachers too - see if they can shed some light on it but also get on same page as to things you want to try.