Update 2: He's coming home today. w00t!
They think he might have a Lepto infection, but they won't know for about a week. That's much better than what they initially thought: ingestion of rat poison, or possibly a bowel obstruction.
This little visit to the ER was a little costly, but hey, you know? He's family, so he's worth it.
Update: I just got back from visiting him, and he's doing MUCH better. It's a complete 180 from yesterday, when he looked like he was on his death bed. When he walked into the examination room yesterday, he faintly wagged his tail before collapsing onto the blanket on the floor. Today, when I grabbed the leash, he was being a total jackass, so I know he's feeling better.
Thanks for the positive thoughts, everyone!
The Home-dog is in the hospital right now, and things aren't looking very good. Last night he started vomiting and had diarrhea. He tried to drink some water, but he threw that right back up, too. He's very weak. We went to see him today, and it was all he could do to even wag his tail a little bit.
The vets are running tests and doing all they can, but they still haven't pinpointed what's wrong. He's lonely, and probably in a fair bit of pain, so, dear Internet, if you can spare a few good vibes and send them his way, I'd grately appreciate it.
Hope for the best!