I have a bit of a headcanon right here, but let me expand a bit on that. :p
I have a feeling that he’d genuinely want and enjoy being a sub in bed. There’s something about the way that his significant other moves above him, telling him what’s okay to do, and what’s not, that really gets him excited. Maybe it’s a subconscious thing, from when he was in the Circus and pushed around by other performers. Maybe, he’s adapted to liking it.
I also have a feeling that if his significant other wanted him to be dominant, he wouldn’t think twice about it, and he’d step up to the plate as best he could.
All in all, I think it’s a torch that passes between the two of them. A lot of the time, it’s just vanilla sex, because that’s what Kurt is most comfortable with, and he really doesn’t know anything else, and when things to get kinky, he has this knack of asking Scott or even Peter about it and getting their intake.
Kurt would never tell you, but Scott and Peter know a little too much about your sex life.