4) Why worry? I know you're a great lover. You make me happy all the time.
Clearly his confidence is not at an all-time high, so a little reassurance that he's not a disaster the rest of the time will boost his self-esteem. And right now, the thing he needs most is self-esteem.
5) If there's something you'd like, just let me know.
Yes, this is pretty close to "Is there something I can do to help?" but the difference is in the approach. Asking what you can do implies there's a problem that needs fixing; asking if he wants something allows him to either articulate his needs or ignore your question. Still, this tactic is a bit risky, as he might feel as if you're disappointed with him. Sometimes, however, changing things up a little will put him in the right headspace and back on track. Just be careful when and how you ask.
Note that in all of these cases, the thing you need to make clear is that you're not mad at him, frustrated or mocking him for not being able to get an erection (or for having lost one).
And remember: You shouldn't take his present condition personally. Trust me, a guy can have performance anxiety simply because he likes you a ton and is trying too hard to make you happy! Or: he could have something else on his mind; some odd thing may have triggered his response; he may have had a lot to drink... any number of other things could be happening that have nothing to do with you or whether you're sexy. If it becomes chronic, there may be a more significant issue at work. But an isolated case or two—or jitters during the first few times you're together—is nothing to take as a bad sign. Male Take: How To Tell If A Guy Likes You
So once you have a handle on not taking it personally, help him realize that he shouldn't take it personally, either. He probably thinks his inability to maintain an erection reflects on his whole identity, so make sure he understands that you know it's a fluke, and no big deal.
Finally, keep in mind that men are a lot more complicated than they're given credit for. It might seem as if they're like light switches—easily turned on—but there's a brain involved, and that can be a wildcard. And when a man does have troubles, he really is at his most vulnerable. So careful what you say and do. He's ultra-exposed—in every way imaginable.