Well, we've got a diagnosis- he's got parvo. (He had his 3 rounds of puppy vaccinations for this stuff!) The vet gave him fluid and antibiotic, and something for nausea. It seems to have helped; he's stopped vomiting and he's eating a bit now, and his energy is a little better. He's got to see the vet every day until he's recovered, so back again tomorrow. Hopefully we'll have him back to shredding my magazines and stealing my socks soon; it's the little things you miss.