Some guys come quickly or prematurely for a number of reasons, I think its more important for you to concentrate on a cure rather then just a reason, this is something that you can do together. john_bees comment is actually a good one, as it will take more time for him to come a second time, but might be a short term release rather then a cure.
There are a number of exercises that will help, most relate to training the reflex to come, or some treatments desensitize the penis. (Maybe free condoms from the clinic. as no man can feel a THING through those puppies)
Here is a website that can help you with PC (Pubococcygeus) muscle training, link
Also its important to talk about it. how and when is a subject for long debate, but if I was your lover I would appreciated it if you could help in anyway. also the treatment ideas are really a two person sport, as they involve you clamping the base of the penis before he comes.