I'm not sure you can just come out and admit that the sex isn't that good without hurting their feelings. But maybe, taking the lead in the bedroom might be helpful. Tell your partner what you like and offer guidance. You can mask instruction as dirty talk, telling him how/ where to suck, lick, etc, during oral sex. Tell him where/ how to touch you. Position yourself the way you like it.
You might even try sex in new places (different rooms, etc.) and you seduce him so that you can take the lead that way too.
Also, if you aren't uncomfortable with it, masturbate for your partner so he can see how you touch yourself and he will hopefully be able to learn without realizing he's being instructed.
If you watch porn together (not for everyone, but some people like it) it gives you a chance to identify something from the movie that you can point to and say, "hey, that would be fun to try."
If none of that works, then I guess you have to be frank...but gentle. Try saying, "I like it when you do (xyz) but maybe you could try it this way..."
If all else fails, I'm not sure you'll be happy together in the long term because a good, mutually enjoyable sex life is crucial part of a healthy relationship.
Best of luck!