Please, stop the freak out. Right now.
Kids all play in different ways. Your son may have more developed social skills and enjoys playing in the ways that girls play, not necessarily 'girly things'. This has nothing to do with how masculine he is. Girls (often, not always) do a lot of discussion before playing: "I'll be the mom and you be the sister and you be the brother and we live in a castle and our castle has a dragon." Girl-style play appeals to a lot of boys who like predictability and enjoy talking during play and using lots of language. It's easy to know you are included because girls are generally more socially advanced at this age.
(this doesn't mean that boys are total lummoxes, either. It's just my experience that many boys focus more on the physical movement, action and horseplay and don't often have a lot of planning or discussion when they play. For kids who like to know what's going on, this can feel uncomfortable.)
My son loves to play with girls. He's five. He doesn't really 'get' the whole "run around like crazy" thing that he sees with boys, because that's what he does when he doesn't have friends around. He doesn't know how to 'enter' that sort of play.
No one has teased him about this. If they did, I would help my son to understand that 'boys play only with boys/girls play only with girls' is a pretty narrow and ignorant mandate. He does try to play with boys, but sometimes prefers the more well-developed stories the girls create. Nothing wrong with that.
One last thing to say: I am raising my son to be who *he* is and to enjoy himself without worrying about what other kids might think. We may get to that bridge, and we'll cross it when it comes. Some kids march to the tune of their own drummer. Those kids can be made to conform, or they start their own marching band because other kids think that they're cool, anyway, just being themselves.