I am not sure what my initial reaction to this would be. I have teen age sons by the way. I won't tell u what size undies I wear because it would not surprise you that no one wants mine. But what would I say to them after the fact, if this were something they were doing....... hummmm, maybe "go to a store and buy some, they have plenty to choose from. You can take your pick." Another thing I might would say is " Please know I still love you (first and foremost) but please stop taking my undies and your sister's undies." I'd be sure to explain about others privacy, you don't go threw his undies, socks, clothes, drawers, books, sheets, anything f his and you would appreciate it if he not go threw your things or anyone elses things in your household.

Would mentioning the things that can be picked up from getting wrong undies from the wrong girl be a way to deter him from taking them from odd places. Lice, there is boungd to be a STD or two that can be contracted that way. If not think up some other skin illnesses that can occur from that. Lice,bacterial infections, strep B, scabies, yeast infections (men do get those to) just to name a couple.

I am sorry you are having to go threw this. Your son is lucky to have a good mom like you who loves him regardless of what he likes. That is very important. Hopefull he will out grow this soon.