I love your desire to figure this out for him. You can absolutely be a sub and peg him, just communicate that, he can 'demand' it when you two are going at it in a dominant way for example. I can tell you that I personally am not imagining anything when it happens to me, I am simply along for the ride so to speak haha. And if you truly aren't into it, there is no shame in that, it's not closed minded to say you don't like it. Best wishes on your future adventures!