I've been masturbating since I was in the first grade. Obviously, I didn't know what it was at the time.
And in the 3rd grade, when I told two female friends I did it, they said they did it too.
By the 5th grade, a large group of girls and I were discussing whether or not you could get pregnant from "having sex with yourself."
I didn't have any sexual abuse issues as a child, but I know now that some of the girls I spoke to at the time did. (I never knew then,) but it's definitely something I just figured out for myself.
The sexual abuse idea is something to investigate, but it could be just normal exploration of his own body.