As long as you don't show up at school drunk or high, I don't think it's really any of his business. If he's the type who would make you feel guilty about it then screw that, you're not the one judging someone, you shouldn't feel guilty. I mean, I'm sure he's a cool guy, I know some of my friends (who I think are cool people) probably judge me for smoking, but just ignore it, don't bring it up if you don't need to (which is most of the time), and let them be the ones who feel awkward about it, not you.