He'll kill the last of the covanent, and the flood, and stop Halos' activation, and then get runover by a group of rowdy Marine driving a Warthog, then it'll fade to black, and say "Don't Drink and Drive".
On a more serious note, I think he'll end up being betreyed by humanity after he saves them, like, execution after the war, or perhaps, killed by someone because they don't think there's enough room in the shielded part of Earth (as it looks small from the outside). That, or, he is killed by Cortana, intentionally or otherwise.
-There's no "I" in team.
-Yeah, but there's no "U" either, so if I'm not on the team, and you're not on the team, NO ONE'S ONE THE GODDAMN TEAM!