I don't know what to do

It seems like my fiancé is pretty much like obsessed with bj's, he's even said that's like the only porn he watches. And I love doing it but I'm so frustrated, I can never make him cum that way, we can go for hours and I just end up irritated that it's not working for him. I do everything I can think of but it just doesn't do it for him, I've even asked him to tell me if there's certain things he does or doesn't like and still nothing. I've realized that pretty much the only way it happens is if we're somewhere that we might get caught. I just don't know what do to, are we just supposed to go to random places instead of being able to enjoy it in our own home? I feel like it wouldn't take very long for that to end very badly ha.