You and your son might have a friendly little discussion to understand each other's viewpoints. You can observe that some people think that "pink is for girls" and might give him weird looks if he wears pink. Your son may say, "Fine--I don't care what they think!" or he may say, "You're kidding--they do? I better not wear pink!"
If your son doesn't care what anyone thinks, then I suggest you support his choices. He won't look back and think that his mother shouldn't have put him in pink. His mother didn't put him in pink. He himself did.
It is very common that small children for a time prefer the colors or habits that are conventionally associated with children of the other sex. I recommend that parents take these choices casually. Rarely, some children maintain these preferences as a lifelong identity. Science does not understand these matters well at all, but it is probably best if parents are supportive no matter what the child wishes to wear.