R.T.
I agree that you need to see for yourself how they play. You are only getting your son's point of view, and like you said, he isn't used to playing with a bunch of kids---so it's really unclear if this boy is just rough or if he is being mean. It's very possible that he just plays rough, and if that's the case, I would try and find a way to pull him aside without your son in earshot (though let your son know you are going to do this) and tell the boy something like, "Can I talk to you for a sec? My son would be really embarassed if he knew I was talking to you about this, so please don't say anything.... He has come home hurt a couple of times and I noticed that you play a little rough. Since you're bigger than him, can you go a little easy on him? I'd really appreciate it. He really wants to fit in as the new kid."
On the other hand, if he is deliberately being so rough that he hurts your son, I would go to the parents.