Retain your gentle grip on his balls, but switch your active hand to a split-fingered grip, using the insides of your middle fingers to rub against the ridge of his cockhead as you slide up and down.
Say: “The way to keep a cock hard is to vary the sensations: to slow down, speed up, or use different parts of my hands to tease the hell out of you. Just by changing my grip I can excite you even more by rubbing against all the sensitive spots on your penis.”