He's not going to get any bigger or stronger at this point, and this is after 4 years of training under Matt Balis' excellent strength and conditioning program (I won't bother with the 1 pathetic year under Paul Longo).
He would have everything to lose by staying another year, since injuries can strike at any time out of nowhere.
That being said, with a continued excellent performance throughout the year, and an excellent Senior Bowl, he'll have a shot at being drafted as a backup QB.