Interesting question. For a mutation to be beneficial, its positives have to outweigh its negatives, over the long run. Throughout most of human history (to a lesser extent today) being stronger and more robust was an advantage for a human. I don't think that advantage is outweighed by potential problems long-term.
It takes a lot of calories (and protein) to maintain lean muscle. The hunter-gatherer lifestyle that shaped much of our evolution has these, but not in a reliable continuous stream. It's more of a boom-bust feast-famine thing.
We store fat for fuel reserves in good times, but in bad times we not only dip into the fat but into the muscle. Some details of mechanism make that necessary, mostly the way we run out of carnitine in the mitochondria during prolonged exercise. You can burn fat for a while in prolonged exercise or starvation, then you have to cannibalize some protein to replenish carnitine to allow your mitochondria to process fat.
That turns out not to be a bad idea, since burning some muscle gives you less mitchondrial mouths to feed in the first place and saves some fat for later. That's the adaptation most of us have.
It's hard to say how the kid's super-muscle mutation would have worked out had it surfaced back in the last Ice Age. The kid might have lorded it over his skinny pack mates, or he might never have made it.