Yes, he was very weak. Yes, if I hadn't dewormed him, the record-setting heat might have killed him. However, the scientifically interesting thing is that he was in better shape than he should have been because of that metabolic slowdown. He wasn't scouring; he wasn't in acute distress, and he very well might recovered without further intervention than the antihelemics. Of course, since the vet did come by, Arthur got a shot of selenium, a shot of B vitamins, and the vet left me some QuickStart to help get his fat content up a bit faster. And we've also got broccoli and spinach for him.
Basically, the expectation is that he will have a very slow but steady recovery from here. Cross your fingers and eat your spinach!