SO bad at oral

So my boyfriend is kinda bad at going down on me, or at least I think he is. Sometimes it's amazing but 9 out of 10 times it isn't. I'm wondering if I'm just not relaxing or if it's him. There's some things he does that I don't like but when I try to tell him he gets mad. How can I bring this up without insulting him and what can I do to relax?