At this age children are curious about the differences in members of the opposite sex. You need to reinforce the message that this behaviour is not acceptable as it will upset the girls and he is not to touch the private areas of the girls. If he is just curious r.e the female anatomy you may wish to purchase a childrens book which looks at anatomy and read the book together. You also need to ask him if anyone has been touching him in this manner e.g child or adult as this may be a posssibility.

I would take him to a psychologist or your pediatrician because my concern would be that someone had innapropriately touched my son... My daughter was molested by a family member which just about killed me - but she was exhibiting hypersexual behavior similar to that.