Have you tried cooked chicken and rice? If you are feeding any other human food stop now. We had a husky with Gluten Intolerance and it took weight off her (mother in law was feeding human food without us knowing, as soon as it stopped the weight went back on). Chicken is always a winner in our house, never gets left.
If you are really concerned go to your vet. A good one will offer you Critical Care Food Tins which are packed full of calories and nutrients. But beware they are not cheap. I'm not sure what brand you have is the US, no doubt it is different to UK