Why doesn't my boyfriend cum from oral sex?

okay my boyfriend is almost 17 and I had never given head before I met him. My first time he said I was a natural but he didn’t cum. and I’ve given him head atleast 20 times after that now. HE STILL HASNT CUM! I brought it to his attention last weekend because I was down there for 40 minutes and I even tried to see if I could finish him off with my hand. and then he told me I could stop and that it was fine. and I was like damnnn you last a longgg time! and hes like I know but its only when im with you that I can last that long. so I was like alright am I doing something wrong and hes like HELL NO your fuking amazing at it. so im said…alright how long does it take you to finish? and he sed anywhere from 5 - 25 minutes depending on how long I want it to take. and so I don’t know why he hasnt cum yet but I think its absolutely impossible to make this boy cum unless we have sex haha, but we dont do that often at all. and im trying new techniques and trying to go longer and changing my pace, using a little teeth occasionally and I don’t know…I have nooo idea why HE WONT CUM! He seems like hes sooo close and then im waiting and waiting and then minutes go by and I don’t know he just doesnt. haha someone please help me because im running out of ideas!