Well, this is an unfortunate situation. But don't feel badly about it.. a lot of women have done it, you're not the first or the last to fake an orgasm during sex. You may want to tell him that you still feel good during intercourse, but oral is what
really gets you off, and just emphasize that. There is nothing wrong with only being able to achieve an orgasm through a certain method. This happens for both men and women. Let him know you still enjoy intercourse and will gladly continue to enjoy that with him, but encourage him to give you more oral... a great way to do so is to give him more oral as well
Just don't beat yourself up over it. But if you feel you can tell him, remember, today is going to be easier than tomorrow, and tomorrow is easier than the next day and so on, because the longer you go, the harder it becomes to tell the truth. You may find when you share something like this that he has had something he wanted to say but was afraid to too.... and this is a great opportunity to bust out the "If there were one thing you could change about us, and there would be no hurt feelings on either side if you share, what would it be?" It is important (for both of you) when you share something like this, that is difficult to say, that there be no retributions when it is shared, otherwise open and honest communication will eventually cease because you will be afraid to say anything.
I wish you the best of luck!