As awkward as it is to talk about a small kid's sexuality, you're probably correct.
I doubt he's ever masturbated or even touched himself because mommy told him that thing between his legs was wrong and defective and needed to go. This is not just some kid whose parents told him that masturbation was dirty and sinful, he was told the whole organ he had was the root of his problems. So, I really doubt he used it for anything aside from peeing and whenever he had any involuntary erection, it might have been a truly terrifying experience.