Don't worry, it's just what little children do, and I completely agree with Andy there. Talk to his teacher if you are that worried. But I'd say, talk to him too! It's nothing bad, so just breach the subject like " You know, at school, there are lods of little girls, yes? Well, playing with them is fine, and I'm glad to see you have so many friends! But, you know there are some things you can't do with your friends, because they will not like it...blah blah"
I haven't got any kids, but loads of cousins whom I babysat...lol