When my son (swcah) was a baby he had hair in odd places for a baby like dark hair over his whole back, it all came out when he got sick and didn’t return untill his late teens. I was told it was because he was missing something and when it was fixed it went a way. Remember every one is different even if they have the same thing and just shareing helps know it could happen but be open not every thing will happen to everyone. Hang in there and enjoy your son.