That definitely sounds like jock itch to me. My husband says if your son has athletes foot, he can easily transfer it up there. Or he could have picked it up in a locker room. I would get the lotrimin cream, which is by the Tinactin powder and sprays for athletes foot. I don't know if it's okay or not, but putting the medicated powder on his scrotum might help keep it dry and treat it as well. With most of the anti-fungal treatments, it's supposed to work in about 2 weeks. If you try them and it's not getting better after that time, I'd go to the doctor. I wish you the best of luck! Hope it clears up quickly! And as an after thought, I don't think 13 is too young for puberty. I started growing breast when I was in second grade. I remember because I had a lump in my breast and I thought something was wrong. My mom took me to the doctor and he told her that it was just breast tissue developing. I needed a bra soon after. I was six or seven at the time (I can't remember if I'd had my birthday yet or not). I started my cycle in sixth grade.