Seriously, this kid is going to have a better life that most of these spoiled blobs I see around lately. And, really, even if the epiphyseal rumor is true, what's so bad about being a little short? Short people regularly outlive tall people without even trying. My kids will work out, too. Not this much, but definitely every other day or so.
And it is quite possible that his systems adapted to the demand for strength by developing such musculature without the use of any steroids. He is at the age where muscle cells are still dividing. His bone density must be excellent if his diet is good. I actually envy the little guy a bit. He is set up for a life of easy fitness, should all other environmental conditions favor it.