Ask your vet about giving him some Pecid AC. Kidney trouble can cause nausea. Pecid takes care of that and they will often start eating better. I used to give Beau half a tablet once a day. Also ask about sub-q fluids. They are easy to do yourself at home and make a huge difference in how the animal feels.

I'm sorry to hear he's not doing well. I hope that he improves soon.