my boyfriend never cums 😒

I've been with my boyfriend a while now and we have a pretty active sex life. It's great, sex is really good with him. But it's always only me that gets an orgasm, I can never seem to make him cum. The one time I did I tried doing the same again next time and nothing happened. He says it's fine and not to worry about it but it does still bother me. I don't know why but it really gets to me that I can't make him cum. Any advice on how to get him to cum? What do you do that guarantees your partner will finish?