Easy answer: Laugh at him. You could also compare him to previous lovers, or get really pissed off at him. Best of all, try icy silence -- he should know what's wrong without being told, the selfish jerk.
Unfortunately, the reason those all work is that he'll never be able to get in bed with you again, so you'll have the opportunity to find someone better. If you're just looking to improve performance on your current model, though, you're going to have to try some other methods.
I'm not a sex coach, so if you're looking for specific tips or techniques, head over to The Big O. I'll tell you how to get him to try those methods without crushing his delicate male ego. Remember that he isn't bad on purpose, and he's not trying to be selfish -- he just needs a little help.
The trick here is to get him to try harder without hurting his feelings or letting him know that he currently sucks in the sack. To do that, all you have to do is keep in mind two basic elements of the male psyche: Ego and General Horndogginess.