Difficulty Having an Orgasm

Does anyone have any tips on helping me achieve an orgasm through oral sex. I have explained to my boyfriend that I am hard to make cum but he is very determined to make me cum from oral sex. He can give me head for like 30 minutes and my legs will shake a lot, but I don’t squirt or anything. He gives VERY GOOD head so that’s not the problem. Any tips will help please and thank you!!!!