[Sin Secret]

Here's the bottom line; do you really like giving him head? I could be wrong, but thats probably a no. Maybe you enjoy giving him the pleasure, but it's likely that giving oral isn't exactly your favorite thing to do. If that's true, than it really doesn't matter if he likes doing it or not. You do it for him so it's only fair that he does it for you.
Vagina aren't the cleanest, tastiest, rose smelling things in the world, but neither is his penis, so him not liking the look, taste, smell, whatever is no excuse.

Tell him you want it, and it means a lot to you, and if he still doesn't want to do it (or does it then but never again like before) than (and this is my opinion) don't give him oral anymore.

This may seem rude or crude, but if you're giving him oral and he's not doing it for you even though you've mentioned that you want him to, than there's an imbalance and I recommend saying simply "I won't do it for you if you wont do it to me".

Many men love giving women the pleasure of oral sex. If your man's not one of them than you need to figure something out.
As far as getting him to want to do it, make sure your genitals are clean (use a low pH soap for your vaginal area and don't shower more than once a day) and make sure he's in a sexy mood before you bring it up. Try flavored lotions and lubricants to change up the normal taste and experience for him. Have a system where he does it to you, you do it to him, and then you have sex in a new and interesting position. Try 69 sometime (but make sure you're both doing your parts). Create a sexual game where you each get one point for every orgasm you give each other (two points if it's in a new, interesting, or crazy way), then at the end of the week or month whoever has the most points gets a sexual favor or act of their choice (within your own limits).

On a side note, if he really doesn't like doing it, find out why. Just straight up ask him why he dislikes it, and maybe you can fix the problem. Communication is very important when there's a subject that people don't talk about often but really want to. If it's something fixable, give it a shot. If he doesn't really know, try the above ideas, and if he just flat out won't do it, you're welcome to be angry/annoyed at him (and punish him as you see fit).

Good luck.