I think it's more a matter of showing him concrete, valid facts and literature about the benefits of cannabis. Two good places to start are the links below:
Erowid Go to plants & drugs/cannabis
Marijuana Law Reform - NORML Go to medical use

With some people, all the talk in the world won't make a difference. But concrete facts and balanced information are a good start. Still, you have to be aware that some minds are completely closed shut on this and other matters, so know that going in. If what you were trying to say above is that he's overcome alcoholism, then also be aware that he may have perfectly valid reasons why he should steer around cannabis. Despite the fact that cannabis itself isn't addictive, folks who're aware of addiction troubles in themselves need to be allowed to make decisions regarding psychoactive substances themselves in peace. There are plenty of addicts out there who can't just get high like a normal person and let that be it, not because of anything inherently sinister or dangerous in cannabis itself but because of their own tendency to want to overuse a substance or amplify its effects with other substances in the quest to get more of that feeling.

So that's a long way of saying give him some facts and educate him, but recognize that he needs to make that decision for himself based on what he knows of his own nature when it comes to substances.