Hi Lysdexic,
For him to have so many partners prior to high school does seem unusual. Where I live this would mean he has had 10 partners before age 14. That is very young to be so sexually active. I just wanted to mention the possibility that he has been sexually mistreated or abused. Children (and someone under 14 is a child in these circumstance, in my view) sometimes reenact things that have been done to them as a means of coping and understanding what happened. Sometimes they become sexually promiscuous as a way of seeking love, because sex and love have been equated in their mind. Sometimes they become sexually promiscuous because sex has no connection with love or emotion.
I would certainly not immediately jump to the conclusion that he had been abused, but it is worth considering, in case he is in need of help. There are lots of reasons why someone may have many partners, and the other responses you have gotten make good suggestions as well.