I don't think it's a problem at all. My dd likes to bring flowers for both girl & boy friends in her class (depending on who is her "best" friend that week). The only times I've been bothered is when I overheard another parent teasing their son & my dd, saying that their son is my daughter's "hubba-hubba" (as they sat, arm-in-arm in the corner, reading a book together). And, now, in the pre-K class, the kids are going the rounds of who is going to marry whom (it changes from day-to-day, and sometimes includes a discussion of whether or not a girl can marry a girl, or a girl can marry 2 boys). I have very open and honest conversations with my dd, but also tell her that she is too young to be worrying about these things!
If the term "girlfriend" bothers you, just remind your son that "friends" are friends, and they can be girls or boys.