cant make him cum with oral

I need help! My boyfriend is uncircumcised and really sensitive about his foreskin. He says it's painful when it goes down and I try really hard to use my hand to grasp the bottom of his penis and suck in a way that it won't move the skin too much. I've never been able to make him cum and we've been together over a year...please help! Any tips or tricks / guidance, advice would be nice!