My nephew is the same age as my older daughter (they are both 9 now). He is in Pop Warner football, the little girls are already chasing him... he is quite the ladies' man, already.
Anyway, when he was 5, we had Christmas at our house. He went to my daughter's room and they were playing in there for a long time while all the adults sat and visited in the living room. Well, out come the kiddos all decked out in Princess regalia - gowns, crowns, wands, feather boas, high heels. And my nephew says, "LOOK AT ME! I'M A PRINCESS!" You can only imagine the laughter and hysterics that ensued. It was so cute and funny. Anyway, we remind him of it to this day (see what nice relatives we are, harassing him like this?). His response? "Well you have to admit, I looked pretty cute!"
Let him love the princesses - it's a sign he will be wildly popular with the girls later on, if our experience is any indicator. ;)