When the human race is forced to evacuate Earth, it leaves behind an aging, osteoporotic millionaire, who finds solace and purpose in the company of an autistic child.

After humanity has evacuated earth because of an impending radiation blast, an aging misanthrope must find solace and purpose in saving an autistic child left behind by accident.

I like Tony's version better because, among other things, the billionaire can opt out; he doesn't have to rescue himself or anyone else. But he chooses to do so even though it will cost him his fortune and perhaps his life.

Protagonists becomes heroes when they can opt out of risking their lives, their fortunes, their sacred honor-- but they choose to opt in (As Katniss Everdeen does in "The Hunger Games" and Peeta Mellark and Meg do in "The Hunger Games: Catching Fire".)

Excellent points by dpg (and others) above. Below would be my take:

'A reclusive billionaire builds a rocket to Mars to save the life of a young autistic boy, who has been left behind after Earth?s evacuation due to an oncoming comet.'