What you are feeding sounds about right to me but every dog is different. It's not unusual for a puppy's growth to slow down. What did the vet say about his body condition? Is he too thin? You should be able to feel his ribs but not see them and the layer of fat on his ribs should feel like a tshirt, not a sweater. Feeding him more isn't going to make him grow any bigger if it's not in his genes.