Feed him numerous times throughout the day and yes a high protein food will help increase lean muscle mass but those foods are on the pricey side. If your dog doesn't have any digestive issues with eggs they can be added to his food as well and might make it more appetizing thus he would eat more and get more casein protein which is ideal for night time feeding because it breaks down slower than milk/whey proteins and will provide protein for a longer period of time while he sleeps.

Curious.... What is your motivation for trying to bulk up a Pitbull?