As Nat said, his body was probably starved for protein and now that it's finally getting it it's bringing up the muscle mass to where it really needed to be all along. That really highlights the need to take measurements as well as step on a scale (or do neither and rely on how you feel or the infamous pant-o-meter). A fringe benefit that he will get is that the new muscle mass will be burning htat many more calories 24/7!