His muscles atrophy and his eyes fail to work due to never being exposed to natural light.
But for a real answer: No. You both are and aren't willing reality into existence. You are part of the all, but you've forgotten this. You have to relearn it through experience in this physical plane. But not only are you willing the rules of this plane into place, so is everybody else in the physical plane. Technically, they're also a part of you. But they're still separate in mind.
If you truly willed yourself into believing gravity didn't exist, you could defy it. But it would still exist.
This child who has been ferally put into the modern world would change nothing. Perhaps if he was raised from childhood on metaphysical beliefs and truths by other people who've dedicated their lives to knowledge, he could have a head start on enlightenment and easier break free from this physical plane.
Ethiopians don't die because they don't realize they can become breatharians, they die because they don't know how to manipulate the physical plane. It's not a natural ability.