I agree with what everyone else has said. It was his time to go, and there's probably nothing you could have done. God would have put you in the position to do so, had you could've saved him.
It's also phenomenal he pulled over.
I know that's got to be really unnerving, and I wouldn't be able to sleep either. Any decent human being would be feeling awful about it, and I'm sure pretty helpless. I'm so sorry it happened.
I really hope God gives his family peace.