So he followed through on his promise and now he's got the mass to log some time under center. To gain that much muscle in that much time is a great accomplishment. He should feel proud.
As a drop out from Kinesiology, and a certified personal trainer (now lapsed) I can guarantee that no one gains 17 pounds of muscle in a summer, even a long summer, unless they are hitting steroids big time, which I doubt is true in this case. He may well be 17 pounds heavier, but about 4 pounds is liquid and 4 pounds is fat. But considering the ration of muscle to everything else, 9 pounds of muscle is going to make him a lot stronger, since it will probably be equally divided between his legs (thighs) and upper body (chest, shoulders and arms).