Some tips for a blowjob that makes him cum?

Hi, all has been well but my bf doesn't usually cum after a bj, I've tried all the tricks in the books - using hands, squeezing and rolling motion and everything but I really want him to cum from that, any tips from the people who have mastered it? Ps-He doesn't really watch corn so that's not the issue maybe

Edit - He tells me he likes the feeling but can't really cum.