heres what you should do.
tell him that that is his uninformed opinion talking.
tell him that cannabis is only psychologically addicting and less than 9% of all users ever expirience any addiction from it at all.
also tell him that if he wants to talk about cannabis then he should be well informed on the unbiased truth and not some propaganda he hears on tv.
heres the link to "the emperor wears no clothes"
jackherer.com
i suggest you study it because i doubt the gy will want to read it so you better remember some good facts, figures and arguments from it.