Boxer won't gain weight

We rescued Nelson a little over a year ago. He is almost 2 years old now. The rescue thinks he was left in a crate that was too small for him and neglected. Since we've had him he hasn't gained much weight. He is about 40 pounds and you can still see his ribs and spine. We feed him purina pro plan sport 30/20 grain free - about 3 cups a day. He will not eat it plain, he is pretty picky so we add some rice, chicken breast, or Dyne (liquid calorie supplement) to get him to eat it. Sometimes he isn't interested and will skip a meal but on average he still eats about 1500 calories a day. His bowel movements are normal, however, I would say he poops more often than most dogs. The vet has ran every test they have and even sent some tests to Texas A&M and everything comes back normal. His demeanor is great, he never throws up his food, he plays with other dogs, everything is great about him except he is skinny. He has muscle in his upper body, but his back legs are pretty weak. I came across Satin Balls and was wondering if you think this will help? Thanks!