Well, maybe just explain to your son that everyone's different, outside and inside. Do your girls have any birthmarks or moles or freckles or anything like that? You could try saying that he has freckles just like his sisters, except his are on the inside. That means he has to take his special pills to keep the freckles from spreading all over the place. Honestly, I don't remember when I first realized that I was "different," whatever that means, and I don't know how my mom explained it to me. That's the best I could come up with off the bat.