can't make him cum??

I'm 1 month far in my first relationship. Things are going very naturally because we were friends for a year. Everything's great except for one thing: I can't make him cum. We haven't had sex yet but I gave him head and he just couldn't cum. The first two times, he was tipsy but the next time he was sober... I don't know what it is, he always gets hard when he sees me and the previous 2 guys, both experienced, I've given head said they absolutely loved it... what should I do? I don't know How to talk about it without insulting him. He's a virgin as well, if that changes anything.