I'd approach it from first figuring out who this character is. Their life so far, their motivations, their emotional state. Let the character tell you if he's the type to cum multiple times.
Fwiw I've cum twice in a row one time so far. It was incredible. We were both surprised. If you've ever done it, it feels different. Consider writing with that experience in mind.
Edit: unless there's a reason a character is a sex god, I don't expect them to be one 🙃