This morning, he didn't want to get up. He only nosed his grain, though he did take a few bites of hay. He's so skinny I can see his hipbone shape through the mohair. But his poop is still solid, so there is still a slight bit of hope. So I grabbed 30 ccs of Goat Goop and another 10 ccs of Red Cell and called the vet. The vet said that I should re-deworm him with Dectomax and keep dosing him every 8-ish hours with the Goat Goop. (There aren't any magic injections that won't do more long-term harm than short-term good.) Unless I want to try a blood transfusion for him (from whom? Dahlia?), that's probably the best course of action.
If he's still alive when I get home this afternoon, then there is hope.
ETA: At 2:30 he is still alive and nosing through the hay that I put in front of his flop-in-the-shady stall-ness. He wasn't thrilled with the Goat Goop, etc. but is ok-ish. I also forced some water into him too. Hydration is important in hot weather...
ETA2: At 6:30 Astor is still alive. At some point before 6pm, he had gotten out of the stall and down the little ramp into the yard, where it looks like he took about five steps and collapsed. I know that it was before 6 because he was wet where the automatic llama sprinkler soaked him. He sucked a few swallows of water out of a cereal bowl for me, and ate a few nibbles of llama sweet feed. When I walked away, he was munching slowly on a mouthful of hay from the little handful I set in front of his flop. I'm not going to move him back into the barn until it starts raining or I go to bed.
ETA 3: CK helped me put Astor back to bed. We gooped him one more time, gave him a little water, and put hay and water under his nose. We locked Constantine in with him for company, and put out fresh water.