i AM 15, and heres my advice.
the kids at his school or w/e probobly overcharge for pot, and pinch the sack, its just so mutch eisier to light up on dads stash, i admit, ive stolen pot from ym dad, and my mom doesnt know either of us smoke. basically sam situation your in. just let him know that you know he pinched your sack, smoke with him every now and then, yah know, he gets an A+ on his math paper, why not give him a bud? there is so many ways to do it, not to mention he wont steal from you again after you confront him.
basically, my dad said...
"i smoke, you smoke, you know that, lets not lie to eachother. i will help you in anyway i can, i dont really have a problem with you smoking, i dont really condone it, but yah know i started smoking at your age too, but its not an excuse to steal. i have no problem giving you bud now and then, but you are going to have to earn it."

me "okay dad i am sorry i will not steal from you anymore, im glad you can understand where i am coming from"

dad pulls a joint he had in his ear hidden under neath his hair, and then smokes it with me, i couldnt believe it, it felt weird at firs but now its all good, and mom still doesnt know (SHHHHH!!!)

just recently i came home with my report card (2.0 gpa, raised it from a .64 gpa, so he was happy) and my dad gave me about .5 gram. i was so happy, i went and smoked, etc. and every now and then hell give me a bud just cause he wants to, maybe i did the dishes, or had been really nice to my mom, etc. it really is great.
just dont put your self in the position where it could end up being a power struggle and him and you argunin in front of mom, yah know how it goes, dont want it to end up like
"YOU SMOKE POT!!!"
"YOU SMOKE POT TOO!!"
"YOUR A HYPOCRITE!"
"GO TO YOUR ROOM!"

because thats noe fun and it could hurt your relation ship with him and make it hard on the both of you..
its always good to have someone to tell me "moms coming home and it reeks like dank in here, better change your shirt, ill get the frebreeze"