Don't get all weirded out over that because he's probably going through a stage where he's extra curious what the real thing smells like. You know he's jacking off while smelling them so now that you know, start making a point of putting them someplace where he won't find them as easy, but still leave some once in awhile where you used to. Check the new place to see if they've been disturbed. Gradually leave them in the old place less and less until you don't anymore. I doubt if your talking with him will do anything except embarrass him and there's no need to do that. If you gradually make it harder for him to find them he'll gradually stop looking for them and he may not suspect you actually were hiding them from him. I don't think it has anything to do with you. Just with girls in general and his sense of smell starting to turn a little sexual. This is no emergency so just take your time and gradually wean him away from your panties. Maybe someday you can put an unused pair there and see what he does, or alternate unused and used but increase the times you put an unused pair until that's all you put there. Good luck. .
Did you catch him doing it live? Like, you walked into a room and there he was sniffing them? How did you react? What did you say, and in what tone?
He needs a girlfriend. Do you know any girls his age that may like him and he may like? If yes, maybe try to get them together somehow.
If that doesn't work out, maybe sit down with him and have a nice 1-on-1 talk asking him why he is sniffing YOUR panties, tell him why it bothers you, and that you two should try to come up with a solution to this problem.
Basically, get him to understand that what he's doing is not only wrong, but it's also bothering you. And maybe help him find a girlfriend.
Get him some of this. One of my college teachers told me about this once. She taught a gender in society class, she wasn't just a perv. If he is looking for that special smell, he would love a bottle of this stuff.