Telling someone to relax and making them feel like their nervousness is not okay usually compounds the tension. :>) Are you sure this is the right guy for you? A cool guy is loving and warm as a woman is getting used to something. They try to be understanding. If he is acting irritated, that would bother me if he were my boyfriend. Otherwise, your own insecurity may play a role here and you are 'thinking' he must feel this way and putting pressure on yourself.
As I stated in my other post, take your time, get in the mood (whatever that means for you), have some artificial lubrication purchased and ready to use (any drugstore has it, KY jelly, astroglide, etc.), and hopefully you can enjoy it rather than worrying you are supposed to 'be' a certain way. That won't help.
Don't put pressure on yourself to please the guy. The guy should be pleasing you so that you don't have to worry about relaxing enough, your body takes over. If he isn't doing that for you . . . that's not really your fault. :>)
Do you think that could have anything to do with it? Maybe since it isn't a romantic relationship it is hard to become turned on? That makes sense to me. Have you tried or have any interest in dating?