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.
I went back to the doctor
To get another shrink.
I sit and tell him about my weekend,
But he never betrays what he thinks.
Can you see the real me, doctor?. The Who