So today, actually, was the vet visit for Cooper.
Indeed, he was dehydrated. Doctor injected him with some fluids, which they do in a big blob on the back of his shoulders, where all the extra dog skin is, and it absorbs into the body from there. He also got a shot of anti-nausea medication, which is the same meds they give the dogs that are getting chemotherapy, so he really should be well medicated there. Also, they took some blood for testing, and as of this afternoon, could find nothing wrong with his organ function (yay!), or his levels in any way (yay!).
Doctor said that we should feed him some bland chicken tonight, which we did, in small portions and with an hour or so between each meal. I don't know when I can go back to feeding him his normal food, in small portions, but I'm thinking about doing that tomorrow morning. Or maybe more chicken because I'm afraid of him getting sick again. He has been drinking more water than he did the last few days, and that's good, because I think he's getting more confident in not vomiting immediately after. I'm really excited about what a full night's sleep will do for him. So, having said that, it's time for us to go night nights.