I think in the end they shouldn't really tell whether he'll live, but just leave something to think about. Maybe sort of in the way they did in Halo 3, but tie up a few loose ends before doing so. I imagine him finishing one final battle and being greatly wounded in doing so. He'll stagger up a mountain, put his assault rifle on his back one last time, and see the whole solar system: all that he's saved, and then take a seat against a rock, letting the screen fade to black. This implies that he will probably die eventually from the wound, but he's way too awesome to be seen die.