Just ask him "well why do people drink alcohol then?" because people are obviously drinking to get a buzz or to get drunk. Why is this acceptable as opposed to becoming intoxicated from cannabis? I find that using alcohol as a comparison really helps put things into perspective. Also many over the counter medications will get you high in large enough dosage and this is very dangerous. Point this out and how it really is something occuring among younger people.

How many people have died from cannabis compared to alcohol?

Really your angle of attack depends on the type of person you are talking to. Attack/Compare their bad/worst habits to cannabis use and it will become obvious that cannabis is really an innocent drug.

Anything like drinking, smoking, compulsive gambling, couch potato'ing, etc etc are all good candidates to argue against cannabis.