Most young men put on a little size in 4 years. Maybe he stopped training for awhile. Who knows.
He's obviously on steroids, and if you think that his dad has the knowledge and the connections for sure, you can see that he doesn't WANT to put on weight.
Maybe he wants to stay in his weight class in MMA, or he simply doesn't like the look. Maybe he likes girls and doesn't want to pose naked for a gay audience.
Ever considered that?