This phase of a child pretending to be the opposite sex will vanish very quickly, by the way. It is certainly okay for the parent to remind the child that he is a little boy if the parent is concerned, but certainly should not make a big deal out of it one way or the other, and certainly not cater to the fantasy that the child is something that he is not.
And it is very normal for other children to ridicule a child if he goes too far with his make believe that he is the opposite sex, in fact I think it is probably very healthy for that child.
***This phase of a child pretending to be the opposite sex will vanish very quickly,***
Back in the olden days it was common for a mom to dress her baby boy as a girl for several years. When the boy got his first pair of long pants was a day of celebration.