Maybe not that he hit him but once he had there's no reason he shouldn't have stopped.
That leads one to believe that the driver is at fault.
Legally, the driver is probably at fault. This happened only five minutes after sunset. I know in my state, headlights are not required until civil twilight ends half an hour after sunset. And it's the responsibility of the vehicle operator approaching from behind to pass safely.
However, a cavalier attitude toward visibility is a stupid attitude for a cyclist.