Go back to your standard full stroke up and down the length of his cock with one hand, but a little faster than before. Cup his testicles gently with your other hand.
Say: “I know that men love blowjobs, but I’m going to show you that a handjob from a woman who knows what she’s doing will give you a stronger orgasm than any blowjob can. You just be patient and let my fingers work their magic on your cock.”