If this had happened between two adults, it would definitely be sexual abuse. If this isn't abuse at what age would it have become abuse?
There are definitely adults who don't understand the concept of consent. (I'm about to give an example of this, so trigger warning perhaps?)
For example, two people might meet at a party. Person A might take home person B who is quite drunk and proceed to have sex with person B. This is clearly rape, but it could be possible that person A might actually believe that because person B seemed to be into them earlier that night, because person B chose to get that drunk, because person B chose to go home with person A, and because person B, under the influence of alcohol, did not say no, that it is consensual, even if it is a little sketchy.
That situation is rape by every definition. Person B might experience that as rape. Person A might not experience it as rape at all simply because Person A did not understand that drunk sex is not consensual sex. Person A's lack of understanding of what consent is doesn't change the fact that Person A performed a rape.
To draw a parallel, in the OP's situation, the boy in question may not have understood that what he was doing was non-consensual, but it still could have been. I don't think that we can define whether or not sexual abuse occurred based on what this boy's understanding of sexuality was, ESPECIALLY considering that we have know way of knowing what was going on in his head. I think it has to be defined more by the experiences of the OP. Basically OP can only answer this question for OP's self. (Which I know is a frustrating answer OP and not what you were looking for, but I really believe it's true.) Trusting your feelings in these situations can be difficult but important. If you feel violated, then perhaps it's because you were violated. If you don't feel violated, perhaps it was because what happened was okay for you. (Which isn't to say that it would be okay for someone else, but these things can be individual...)