There are a lot of good foods out there and even more sub-par foods. Like performanceknls, I'd feed the pup something under 30 percent protein. And as far as ingredients go, stay away foods that are full of corn/soy/wheat, these are fillers. A lot of cheaper food brands are chalked full of fillers and that's bad. My dog is allergic to most grain, so I have to feed grain free food, food allergies are something you may have to deal with, but lets hope you dont. When they're allergic to food, they have gas that is ridiculous, or atleast my girl did! haha I feed my dog "Professional Chicken and Rice Low-Fat". I feed her the low-fat formula because she has a hard time digesting high fat food and it makes her really gassy.
And as far as "bulking" up your dog, the APBT
is not supposed to be bulky like a lot of people think. The APBT
is a lean well muscled dog that is highly agile and able bodied. Like was said above you don't want to push a puppy to hard when working it, because you can do more harm than good. Just start with walking and playing fetch. When you play fetch your dog will build very lean muscle. When your dog gets to about 15mo, you can start doing weight pulling, you can start earlier, but if you do you should do very light work, don't want to damage the still developing joints and bones.