Hmmm, thats a tough one! Poor Andrew!
We had a similar situation with DSD a few years ago, but that was at school rather than preschool. She came home with legs covered in bruises where some older boy had kicked her and when I went to the school to complain about it I was pretty much told that she could be "cheeky" and probably was winding them up
Not happy! But then we had no end of problems with that school...
I'd say talk to the teachers, maybe first on your own to give them a heads up, and then with Andrew so he knows he's got a 'safe' person to tell if he gets bullied again. And maybe make it one of the other teachers you approach rather than the one who's son might be involved coz that could be a bit tricky.
Best of luck, let us know how you get on!