This is far more than just the treatment he might benefit from in the US. Here's the facts. 

THE DAMAGE IS ALREADY DONE. Nerve cells have died of starvation from lack of energy due to the defective mitochondria he had. They aren't coming back. He's got systemic damage throughout his body as his cells have literally starved. So, if this therapy is successful, he will never live up to the potential he might have had he had normal mitochondria. What does this mean in real-world terms? 

Continuing healthcare, throughout his life. Reduced or even zero productivity. A net drain on society. Unless the parents are absorbing this cost all by themselves, this means society is paying for this person his whole life. That's not single payer. Here in the States, your insurance costs would go up if you were using the same company he was. That's how life works. 

The parents need to let go and let him die in peace. That's the real problem here.