Is it normal for my boyfriend to not always cum?

So I have a boyfriend and we’ve been dating for a couple months. I didn’t have a lot of experience before him so I’ve kinda been learning about sex and intimacy with him. We do have sex fairly often, usually(but not always) we’ll have sex at least once every time we hang out. Initiation goes both ways as I think we both have pretty high drives. Anyways, a decent amount of the time he won’t be able to cum. He calls it “going numb” and just says it gets less sensitive over time. At first it made me feel really bad and that I was doing something wrong. He has explained to me that it really just depends on many factors if he can cum or not. (Sleep, hydration, excitement, etc.). I’ve told him if he’s holding back to last longer that it’s okay as well and he can go ahead and cum whenever he feels it. (I prefer only like 10-15 minutes anyway since I kinda get tired if we go for a long time). I’m hoping he’s not lying to make me feel better? Has anyone else experienced this? Thanks :)