Don't leave a stupid note or talk to him or anything, just keep checking and see what happens with the bag. Even if he does end up smoking it, don't confront him about it. Not all toker-parents approve of their child smoking. Its like saying "hey, my parents are alcoholics so they won't mind me drinking!" but some parents are cool with it. I'd say if you let some time pass and keep checking on things, like missing weed from your stash, and if it seems like he's become a stoner, then maybe do something subtle about it and see how he reacts, if he seems cool about it, then you could probably get away with becoming stoner buddies with him.

But thats just my 2 cents.