He's probably just getting alittle tired. When new, many times a day is easy. Back when I was 16/17, I'd pull 5, 6, 7 as many sessions as I could get in. After awhile, it becomes somewhat stagnant. I'd compare it to this: You experience a brand new flavor of ice cream that you just can't get enough of and gorge on it non-stop. Eventually, you tire of it and don't want it anymore. But, if someone else is at the controls and gives you just enough of of it here and there, you'll never lose the craving and always desire it. Same priciple.
On the other hand, if YOU need it so many times a day and he just isn't willing to provide, I'd consider that incompatability, and it's time to look elsewhere. Sexual compatability isn't the only factor in a relationship, but it's important enough of one to be a deal breaker IMHO.