First of all, he should NEVER play with your girls, even under strict supervision. Rats can "do the deed" very quickly and you might not even be able to stop it. So unfortunately the little boy has to be completely alone until he can be neutered.
From what I heard how early they can be neutered depdns on how experienced your vet is and what he/she is comfortable with. So I'd ask around and see what your local vets say. Neutering is a fairly simple procedure and you shouldn't worry about it as long as your vet is experienced.
CongRATS on your first boy tho! I started out with girls but now I couldn't live without my boys, they're such mama's boys!
