That's really nice of you to be concerned with your young friends health but there the important thing....the decision to quit smoking needs to be his. Be there to support him when he finally makes that decision but you can't make it for him. And when he does decide, suggest he look into hypnotherapy to quit. That might tweak his interest and he'll look into it.
You can't convince someone to do something if they don't want to. Even of he does end up quitting for you, it won't last unless he wants to do it for himself. Being 22, people are well aware of the dangers unless they live under a rock. If you keep pestering, it will just annoy him or make him lie to you. Just be supportive of any moves he makes in that direction.