I have a 4 month old American Staffordshire Terrier.. he is 30-35 lbs and eats 3x a day and an hour of exercise a day.. he looks thin!! I can see ribs and hip bones!! He's utd on shots and is free of worms.. he eats Rachel Ray's Nutrish, which I was told is certified as puppy food.. should I be worried and take him to a vet, or is he just going through an awkward growing puppy stage?! We have even started mixing wet food every other day with his dry food...