i dunno man. usually parents are ALWAYS right. but here what i think: u just try to not give him reason but not to prove your right. just let it as it is. maybe tell him you like it, and that all those diseases are due to abuse. just like alcohol. so i guess u tell him u wont smoke during school, only weekends or whatever just so he can chill a bit. after few months hell just be used to the fact u smoke weed and maybe hell shut up.

or u can always search for articles proving the benefits of marijuana use. and if weed has more tar u dont care coz u dont smoke 25 joints a day.

anyway i guess thats it man, if hes really closed minded and all i guess u cant argue. so let it as it is...