Well, I talked to the vet, they gave him fluids & sent blood for testing yesterday. Blood tests came back today - She said it looks like his kidneys are not working as well as they should. She also said he had liver enzymes elevated, so it could be more than one thing going on. Unfortunately, other than food changes (for less protein,more fluids) they can't do much for him. She's going to teach me how to give him fluids subcutaneously (sorry if its mispelled) at home. He really perked up after they gave him fluids, so I hope I'll be able to get the hang of it. I feel so sad. Don't have any idea how long he'll be around, and I just want to make his life the best I can while he's here. Thanks for your suggestions, everyone.
Here's a picture of us-