More sex! TMI

I want to have sex 2 sometimes three times a day but my husband says he's worn out. He's a fit athlete type. It's not like sex is more strenuous than a two hour work out at the gym. Most of the time our sex lasts about 30-45 minutes. And I've started taking charge and changing positions to be on top so I do most of the work. What else can I do to help him get his stamina up?