how to teach him how to eat me out??? help!!!

my boyfriend has only eaten me out like 4 times and he says he really enjoys doing it and he could do it forever if his tongue didnt get tired but he gets scared about doing it because he cant make me cum...he has only done it twice to two other people and it was really short. how do i show him how to do it??? i try to explain it but its hard to make sense:/ he really wants to make me cum but he feels like no matter how much he does it that he cant and he thinks hes bad at it. someone please help!!!!