It wasn't an intestinal bug
I got the cat off to the vet's office on Friday, and they immediately determined he was suffering from a severe urinary tract infection, which turned out to be the root cause of the nausea and lack of appetite. Simple enough to treat with antibiotics, but given the lack of food and water over the last few days, they suggested I leave him there through the weekend for further treatment, rehydration, observation, and recovery. That sounded like a wise thing to do so I agreed. As part of the overall treatment they did urine and blood tests, and they found some numbers in the blood test indicating Charlie is in the early stages of kidney failure. And though the vet didn't come straight out and say that when we were visiting the patient today (I took the kids up to visit the 'cat hospital'), the discussion he and I had sounded very much like the beginnings of a management plan for a terminal illness.