2. He's stressed. Stress can be a major boner killer. And it's not exactly something twentysomething guys think to look out for. But if he's had a rough few weeks at home, or he's dealing with major issues, it can absolutely affect his performance. He could even be freaked out about the performance itself. Maybe he's so worried about impressing you he basically gave himself the sex yips. In cases like these, just try and get him to relax and take it easy.
3. He's got a preexisting medical condition he isn't volunteering. There are obvious conditions, like a history of erectile dysfunction*, but a variety of other conditions can impact his penis: heart disease, diabetes, MS. Even certain surgeries can wind up affecting his ability to get erections. There's no immediate solution here without getting a doctor involved. *It's important to note that erectile dysfunction isn't a problem in and of itself. That is, you don't just "get" erectile dysfunction. A variety of other issues can lead to it, but it doesn't just happen on its own.
4. He, uh, prepped for this a little too hard. He could've been really nervous and masturbated a few times beforehand to make sure he'd have some stamina. But unfortunately, he left nothing in the tank. His plan backfired and now his penis has nothing left.
5. He's holding off to show off. This guy just thinks you're into it. When you can't have anymore orgasms (or you just have other things to do), let him know you want him to come. If that still doesn't do the trick, be more firm about it. If he still thinks he's a sex god for breaking records even if you're not into it anymore, tell him he has five minutes to come or you're leaving.
6. His penis has very specific needs. Penises, just like people, are creatures of habit. If he masturbates a certain way, or at a specific speed, his penis could be so used to it that it's not finding what you're doing as stimulating (it's commonly referred to as a "death grip"). Either he grips his penis way too tight or at an angle, or stimulates the head of his penis in a way a vagina never could. It's also possible he's just gotten used to certain positions or angles in much the same way. In these instances, he needs to dial it way down on the jerking off when you aren't around. Or, you can just swap to his favorite position when it's time.
7. He's on new meds. If he recently switched medications for whatever reason and he swears he's never had this problem before, there's a good chance this is the culprit. There are countless medications with countless side effects and having trouble with erections is a pretty common one. Even if he's positive his new meds are the reason behind his performance issues, he should talk to a doctor before changing over or stopping his usage. His overall health should come before his orgasm. And, of course, even over the counter medicine can affect him down there.
Hmm... So, what do you think? When thing get too hard in the bedroom, maybe both of you should chill up a little bit and don't forget to always use condoms and play it safe!