Acting Out

So hoping that I don't have a future bully on my hands. Matthew has been kind of naughty the past 3 days at school. He's been pushing/pinching. Not sure why he is acting out like this. In a way it's a good thing because he's becoming more social, but of course I don't want a little bully for a kid either. So hopefully with a little discipline with nip this in the bud real quick. Not sure if he's doing it to his only friend from early childhood or what, but the kid is big and could knock him on his butt if he wanted I suppose. Then again, I do want my child to stand up for himself if he is picked on....