I don't like Hills products, but when animals need a highly digestible high calorie food, Hills AD is often does do the trick; ask your vet about it. But do NOT keep the dog on it once he is up to weight; it is good for putting weight on but is not a high quality food.
I'd seek out a good quality maintenence diet such as Evo, Innova, Wellness, Canidae Grain Free, etc. You can also supplement/enrich your dog's diet with organ & muscle meat for some added calories, fat, and protein. Just remember that any dietary changes should be gradual and carefully monitored to prevent digestive upset.