Sex Q&A: Is There Anything I Can Do to Make Oral Sex Less Painful?
No sexual activity should end up in prolonged pain -- unless of course you're both consenting adults who love S&M! Yes, from time to time we all have to put up with a little bit of discomfort while our partner finishes off, but definitely not what you're describing. Most people have heard of "mouth cramp," so explain this phenomenon to him if he hasn't. Unless he's a selfish bully, he'll understand if you say that you'd like to use your hand for a while and give your mouth a rest. Oral sex can be all the more exciting by varying the stimulation.
Nestpert Dr. Pam Spurr, author of Make Love All Night & Talk to Him in the Morning and Naughty Tricks and Sexy Tips: A Couple's Guide to Uninhibited Sexual Pleasure