When you think his arousal has subsided a bit, start the long, slow strokes again. Remember that he was close to cumming just a little while ago, so he might not take long to get back to the edge. To hold him off, try varying your strokes. For example, just use your fingertips on the head of his cock, or fondle his testicles, and then go back to the long, slow strokes. As you do so, describe for him what you’re doing.
For example, you could say, “Now I’m going to pay some attention to those balls of yours. I’ll bet my hands feel warm because of all that friction against your cock. I’ve heard that some men can come just from having their balls tickled, but don’t fret; I won’t let that happen to you.”
Then, go back to the long, slow strokes.