First, he wasn't 8 years old. He was almost 10; that is preteen.
Second, you are naive if you believe 9/10 years old, or even 8 years old is too young to think about or know anything about sex. Very naive. Children at very young ages, preschool and the like, are very curious about their bodies, hearing and repeating things they have heard about sex, experimenting in the common first games of sexual play called "Doctor", and sharing with their friends what they have learned from media, siblings, etc.
My son (now 13) told me that when he was 4, a little girl at his daycare told him that she liked him, and had asked him to go to a corner one time with her to express those feelings. She told him to take his pants down and she would take hers down and show each other, because that's what people who like each other do... Now I certainly didn't teach him that, and he was not molested by an adult or other person growing up. But whatever that girl was exposed to in her surroundings, she repeated to my son.
My son comes to me whenever he has a question about sex or his body, or when he overhears things at school. Now that he is in middle school, he says the 8th graders are constantly telling sex jokes all the time, the boys have a 'game' called 'slap



Friday' where they harass the girls, and he often sees boys pin girls up against a wall, and sees other affectionate and inappropriate things going on.
Kids learn from what they hear and see around them....even if they didn't get it from their parents, who usually are trying to protect their children from such exposure. It doesn't work that way. Unless they are very sheltered, they will be exposed to things like that on a regular basis. It's part of life, part of our culture.
Now, my son is also very immature...more like 13 going on 9....but that doesn't mean they are not thinking about girls, or curious about sex, or want to try what they have heard about from other boys, etc....in fact, that is what often gets them into trouble with girls because they don't respect boundaries and/or don't know how to interact with them appropriately. Trust me, I have firsthand experience with this, and have gone through several situations with my son as such, simply because he is too immature to think for himself how his actions affect others, and that you don't pop girls' bras, or slap their butts just because they think it's a big joke or a game....I've been dealing with him repeating immature and inappropriate behaviors such as these since he was in 5th grade - that is 10 years old.... (However, he does not like to go to Hooters nor watch movies where guys and girls kiss because he says it makes him uncomfortable...). It is exactly the immaturity and curiosity that gets them into trouble...
I guarantee you a boy almost 10 yrs. old was not completely innocent and was not uncurious about sex and experimentation.