I'm a male, and I'm saying that up front so that if you hate that I'm in this thread you can ick out and stop reading now (sorry, in that case). Now that I've gotten that out of the way...
This isn't easy to admit outright, much less publicly, much much less on a forum intended for and very well populated by women. So, please bear with me. If you can handle hearing about the topic, you might talk to him about his history.
From the time I first became sexually active until nearly the point where I turned thirty, I was very "giving" where oral sex is concerned. It wasn't even just for the women I was with, but for me too -- I enjoyed doing it.
Every single woman I was ever with hated reciprocating. I didn't feel like this bothered me, and I really just stopped even asking about it. Then, one day, I got to thinking, "Why isn't that an equal kind of thing? What's it like to be able to just relax and feel good?"
At that time I was not single and was not with the woman I am with now. When that thought occurred to me, it didn't bother me. But when I brought it up, my girlfriend took extreme offense to my bringing it up. Now, I had long since gotten over that things were unequal in that department and I understood that the physics involved is much more uncomfortable for straight women than straight men.
But what I wasn't prepared for was to be yelled at and called names for bringing it up. She, at that time, wasn't the most emotionally stable person exactly, and I knew that (aside from the predictable spans of time) there were random days when nothing confrontational should be said. And I didn't mean to be confrontational. Plus, I didn't know it was one of those days.
It changed something about the experience. The fact that I was yelled at and belittled for daring to bring up equal attention in the bedroom utterly destroyed my enjoyment of an act I had been quite happy to perform my entire adult life to that point -- with her. I might have forgotten and that could have passed, but then she was angry and pushy when I didn't want to do it. That made it stick.
Some months later, I was single again (lol -- not really, but in the context of this topic lol, right? ugh... excuse me...), which always gives one the chance to reflect. And I let myself think about how unbalanced that aspect of the bedroom had been my whole life. Losing enjoyment of the act with her became losing enjoyment of the act, full stop.
That never changed. I'd change it back if I could -- if there were just some switch to be thrown. I'm not even sure how.
But the point I'm getting at is, there may be some experience driving the imbalance. It might not simply be an unwillingness to reciprocate, and after two years with you, it's probably safe to say that he cares about your needs too. There are some people who have always simply been turned off by the idea, and maybe he's one of those -- if so, you may not be able to change that. Yet if it's some past experience, maybe it's something you could help him past or maybe time could change it if you're persistent but gentle with communication about this (ahem) at the right moments.
As always when commenting on complicated or sensitive matters, I have absolutely no idea if this helps. But good luck either way. I wish you both well.