He's not coming. Am I doing something wrong?

Okay, so my SO and I have been together for two years, started having sex about 6 months ago. We've been friends since 2008, known each other forever. I cum multiple times during sex, but for some reason he rarely cums. I don't know if I am doing something wrong, or what. He says he's happy but he will only cum like 1 in 4 times we have sex... he always makes sure I cum. And I feel guilty he's not able to reach climax... what can I do?