http://www.drugpolicy.org/marijuana/factsmyths/
http://www.erowid.org/plants/cannabis/cannabis.shtml (the 'Myths' section may help)

Tell him he can't prove cannabis had anything to do with your grades and try to bring them up, and that not all jobs are dependent on drug testing.
Also if you don't plan on quitting you could try to tell him it would be safer to smoke in the privacy of your own home than somewhere else where there is risk you could get caught.
Good luck, im not doin any of this until I turn 18 or I really have to.