IIn during a blowjob?

Almost immediately after you cum your penis goes soft. It's kind of hard for another person to blow a soft penis, plus not all people are into eating cum. The entire point of having good sex is to keep it going for as long as possible. It's not that he doesn't want to eventually, he just wants to draw it out longer. You're giving good head, no guy in his right might wants to cut that short. If you pay close attention I'm sure he's giving away some subtle indication (probably a slight facial expression or maybe even a twitch).

A lot of guys associate how long you can go without cumming with masculinity (which kind of make sense since it's a bitch to hold it in). There's a condition called premature ejaculation that affects a whole lot of guys, he could be one of them. It's basically where you cum as soon as you get really aroused. Most guys that suffer from it are extremely self-concious about it so don't bring it up.

He might get better about it with time. Like anything else, it just takes practice to get good at.