Wow! Your seeking the holy grail. If kidney issues are indicated the diet will be restricted to a food like science diet KD, which is about 90% ground peanut hulls. Hard to gain any weight with that. If it's not necessary to restrict diet due to kidney function, I use ground beef and brown rice mixed into a high quality dog food. If the dog is severely debilitated, try to jump start the digestive system and immune response with cultured butter milk, or yogart, or whole milk, unpastuerized, (with milk leave out in a pail till it turns a little yellow and there are dead flies present). I am not kidding, this was taught to me by an old race horse vet, as a solution to digestive issues brought on by illness. It works on dogs too! Literally seen it bring horses and dogs back from the grave. If the kidneys are an issue, they will usually return to function in a reasonable period of time, but will require some rest, hence the peanut hull diet. My guess is weight loss due to the antibiotics killing all the beneficial flora in the gut tract, resulting in upset, and possibly a blood chemistry thats a little off, probably potassium. Repopulate the gut tract with good bugs, encourage appetite slowly, the blood chemistry will solve itself. Little bits at a time don't overwhelm him, stop while he's still hungry. I am not a vet, I am only an old hand at doctoring dogs and horses. I suggest possibilities based on advice I have been given over the years that worked for me. Maybe save you some learning curve and expensive trial and error. Good Luck.