This is unfortunately a problem HE is going to have to work on. He needs to practice masturbating so he can tell when he is past the point of inevitability so he can STOP before he gets there. In the meantime, he should be helping you reach before he does via oral stimulation or whatever else can do the job. If he refuses to do that, then I think his problem is one of selfishness in addition to a lack of self control. I might also suggest that if he isn't wearing condoms, he should be... nice THICK ones. Not only will he be protecting both of you from possible negative consequences like pregnancy and STDs, they will also help to desensitise him.
BTW - you aren't one of those girls who have been pretending that you enjoy the sex, are you? If so, you need to stop it. He needs to know he isn't doing the job so he can improve. You can be gentle, but don't lie.
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?