I think what would make it abuse would be two considerations. First, was there a big difference in age? As you were 13 and he was what?, 17, that's a big gap in both age and maturity. He was old enough to know this was wrong and that he was taking advantage of you and using you. The possibility of innocent experimentation isn't on the table for discussion here.
Yeah, thats what I was thinking. He was approximate 4 months shy of 17 when it started, when it ended he was about 3 weeks shy of being 18.
Second, did you have a real option of saying "No"? That doesn't mean just the ability to utter the word. The question is one of whether you could have said no without suffering consequences that would have been unbearable to you as a boy: rejection by a cool older sibling, withdrawal of opportunities to play together or be included in activities, denial of love and affection, threat of violence, shaming or exposure, and so on.
I don't know exactly what the "consequences" of saying no would have been but I do know that I most certainly looked up to him, I know that my parents had recently (within the last year probably) gotten divorced and my father had gotten sick. 2 of my granparents had died and my mother was going to graduate school to get her masters degree, so there definately was an emotional void there. Also I remember when I was thinking about disclosing to my mother, I had this thought of "If I tell he's going to kill me". So there had to be something going on.