I would say it is more than likely a result of his man handling his dick while masturbating. You could exorcise your brains out and never tighten your sphincter enough to compensate for that. Especially if he is spankin a lot when you aren't available. Hard as it might be (pun intended) make him swear off his dick for 2 weeks. By the time that's up a slight wind could probably get him off. Then slowly start increasing the frequency you have sex. He needs to recondition the level of friction it takes to get him off. Just to keep it interesting, and yeah lol a little more torturous, you might start edging him as well. Maybe throw in some ass play for HIM.
I was a "DEDICATED" top until just this past year. Bad experiences early on combined with my love of fucking ass and the fact that most guys took one look at my junk and wanted to ride it all converged to keep it that way. This is the first "real" LTR I have had, and he was the first guy I really trusted. We started off slow, toys etc, and without all the gory detail I can honestly say I am fully verse now and can't believe I waited so long. It opened a lot of new doors in our relationship as well.
Just friendly suggestions. 😉