Opposite problem from most couples... It is nearly impossible to make my SO cum. Advice?

I'm 19, he's 20. We are pretty sexually active and adventurous, confident, completely healthy, in great shape and considered fairly attractive. He hasn't jacked off much since high school, so it isn't that. But it is nearly impossible to make him cum. We've had sex for two hours straight. Nothing. I gave him a 45 minute hand/blow job. Nothing. I've made him cum twice, after a mix of sex, blowjobs, handjobs and teasing that lasted several hours.

He's been checked out by a doctor. Nothing is wrong. He's more than healthy, and in fantastic shape. But I just can't make him cum, and even though I know he loves making me cum, I want to do the same for him. It's like a little personal victory every time he does, and I love the look on his face when I've done that to him, and I want it to happen more. I love making him feel like that, and teasing him so close to the edge (and being unable to have quickies) is really weighing on me. Advice?