I would be disturbed by this if I were in your position. I would feel like my privacy was being violated and I would disprove of the stealing too. I guess this must be a sensitive issue for you as well as him. This is private to him, but he doesn't seem to keep it that way. What I think you could tell him straightforward is that you know he is taking your panties and that it's not okay to do that, that they're your very private property and you are bothered by it. It would be useful if you could get him to buy his panties rather than steal them - maybe from a second hand clothes store if he wants them to have been worn, but definitely no stealing. I'm not a mother but, if I were you, I'd find it very difficult and challenging to bring up such a topic with my own son in a manner that would be appropriate.