Instead of telling a guy that he "sucks" in bed, which will come out rude no matter how you say it, how about you work with him? Say you want to try some new things, or get a little more creative? That way he can learn, and so can you, without one of you getting their feelings hurt. As you should know, people perform differently when it comes to sex. Thats part of what makes it what it is.

(And by the way, before you post try to put your post under the heading that makes the most sense. As far as I can see this doesn't have anything to do with sexual literacy)