As he appears to instantly develop a hellacious underbite to go with his suddenly-pointy-carnivore-type-tearing-teeth protrude forward from his mouth I'd say his jaw definitely DOES change shape.
Fair, and nicely put, point on the "WTF?" sensation that would probably accompany suddenly REALLY acquiring powers. I do think the "bonding with a magic stone" origin probably handwaves away any problems like that!