Good job in posing this question. The general requirement for it to be considered abuse is for there to be a power disparity between the two persons. Often that disparity is due to an age difference, but it doesn't have to be.
You describe him as always initiating it and teaching you what to do, and your going along with it because you were lonely. He may have just been a kid wanting to experiment, but he used and manipulated you to do so, and that seems to fit the definition.
Seems I've got to have a change of scene
Every night I have the strangest dreams
Imprisoned by the way it could have been
Left here on my own or so it seems
I've got to leave before I start to screamJoe Cocker