This guy sounds like he's past the point of oral help. Putting weight on is not the first concern for this animal. Rehydration and stabilazation is. Now as I've said already, oral fluids aren't going to work as the body will be in absolute survival mode. Electrolytes, nutrients, calcium and other vital elements are drawn from the tissues to try and keep the blood levels stable. The blood is directed to the vital systems only, that being the brain, heart, lungs and kidneys. Thus the result is the gastrointestinal tract is not up and running. To get around this, parental fluids can be given intravenously or intraosseously. Once blood values and hydration have stablized to normal, feeding can be slowly attempted. Once the animal is eating on it's own normally, then you can worry about fattening him up
Your vet will help you up until this point, good luck!
Ian