There are a few high calorie products on the market that are designed for sick/weak dogs. I think they have a large amount of sucrose and protein concentrate and are easily digested. I gave one (called 'STAT') to my old girl, Reddy, when she was recovering from her chemotherapy treatments and couldn't eat anything. Its a liquid, so I could get it down with a syringe. I ordered it from a catalog. I'll try to look up which one, if anyone needs it.
Marjie