Stop feeding him Purina or any of the other junk dog foods, even science diet is bad. Buy wellness instead or solid gold. Read the labels and avoid foods that contain grains and meat by-products. Dogs cannot digest grains and by-products are the crap of the animal (brains, tendon and such)
My puppy at 12 weeks was around 20 pounds. She was 30 pounds at 4 months and now 54 pounds at 6.5 months. I agree that your puppy should get a better food.
Hard to be totally accurate since boxers have a range in adult size but my almost 9 week old pup is 10.5 lbs.