You don't. You make him go to the doctor, not because you need sex that lasts longer than a commercial break, but because a sleepy trouser snake is probably just a symptom of something worse. I'm not out of sympathy with your man, because admitting you've got penis problems, even to a medical professional who isn't going to laugh at you, is uncomfortable and awkward. It's difficult for most guys to go to the doctor in the first place. But it's gotta happen.
The penis is much like the canary in a coal mine; if it droops too much, that's not good news. A limp wand can be caused by a lot of things: stress, cardiovascular disease, venereal disease, aging, and so on, and that's just the physical symptoms. We're not even getting into the psychological, which can get even more uncomfortable.
But, one way or the other, his body is telling him something, on some level, is wrong. He needs to man up and face it: that simple.
I know you care about him, otherwise you'd be gone by now, but you gotta stress to him that this is a deal breaker for you. He has got to go to the Dr and figure this thing out, or you go out the door. Believe me, I was married 13 years to a man like this and sex dwindled to ONCE or TWICE PER YEAR!!! Do you want a sexless relationship?
I know from experience about the two pump chump thing. That is a good thing, it means he is not cheating on you, my wife and I have sex about once a month, I always tell her since I barely get it, im so excited to get it I am done as soon as it goes in. If you want him to last longer it has to happen alot more often.
Maybe he's not that horny. Or not that into you.
Sorry. But it's possible.