Rule of thumb is if you rub your hands down his sides, you should be able to feel the ribs just a little bit without much pressure, but not really see them. It really depends, I like my APBT's on the thinner side.
Also, it really has to do with the dog. My personal dog, has a beautiful tuck, and is nothing but muscle. You can feel the ribs, but not see them. My foster however is SO high energy that no matter how much food and conditioning we are doing to her, you can still see the ribs.