*Bad advice follows*
Some people genuinely aren't aware they they suffer from halitosis, in which case it isn't really their fault, is it. So they go through life becoming more and more hurt through alienation because none of their "friends" had the courage to say something.
Put yourself in his shoes....how would you feel if you were the only member of the circle of friends whom everyone avoided kissing goodnight. You wouldn't feel too good, would you.
There's no easy way to do this. It's best done by a member of the group whom this guy likes best - the one who he knows wouldn't deliberately hurt or offend him - and they need to tell him straight.
Better that, than he falls in love with his dream girl, and she runs away because of his breath problem.
And he'd never know why, would he!