in my experience a person who thinks so is impossible to convince. No matter how hard you try, all the same it will all boil down to "start using drugs" or "dumb drug addict." It's better to just try not to let him know about your hobby. And you can also very carefully and slowly say that marijuana is used for medicinal purposes, in many places it is legalized and that alcohol is recognized as the most dangerous and widespread drug, that there is a serious dependence on alcohol, but not on marijuana.
Let him read these articles.
https://rastamaster.info/1373-bolshinstvo-doktorov-ssha-za- ispolzovanie-kannabisa-v-lechenii-detej /