Take a deep breath, MISSCLEO, or take two — take however many you need until you’re back in touch with your inner mom, the one who doesn’t freak out.
Your son may be a cross-dresser or he may be trans or he may find bras and panties titillating because women wear them and he wants to sleep with women (not be one). (Lots of gay boys are titillated by jockstraps — but a closeted gay boy can collect ’em all without freaking out his mom.) We can’t know whether your son is a cross-dresser, trans, or merely titillated, MISSCLEO, but he’s clearly exploring and wants to do so privately. So while he could go to his mom and ask for a pair of panties and let her know exactly how he intends to use them, he doesn’t want to ask his mom for a pair of panties or share his uses for them with his mom. He knows you’ve always accepted him for who he is (but a reminder never hurts), so if this is about his gender identity, well, you’ll have to trust that he’ll share that with you when he’s ready. But if this is about a kink, he may never share that info with you, because why on earth would he? Kinks are for sharing with lovers, not mothers.
Give your son some space, including the space to make his own mistakes. As teenage misbehavior goes, swiping a single pair of panties isn’t exactly a crime spree. If you suspect he snuck into the girls’ locker room and made off with a bra (there has to be an easier way for a guy to get his hands on a bra!), you’ll want to address that with him — not the “Why do you want a bra?” part, but the risk of getting caught, suspended, expelled, or worse. There are too many prosecutors out there looking for excuses to slap the “sex offender” label on teenagers — especially in the Bible Belt.
My hunch is you don’t have a sex offender on your hands or a kid drifting into organized crime. You have a slightly pervy teenage boy who’s curious about sex and who may, like millions of other men, have a thing for women’s undergarments. You should emphasize the Not Okay-ness of shoplifting panties from stores or stealing bras from classmates (or the siblings of friends or Laundromats or thrift stores) and the possible consequences should he get caught — theft charges, suspension/expulsion, losing friends, coming into the sights of a sex-negative prosecutor. (Seriously: A man like Harvey Weinstein gets away with assaulting women for decades, but prosecutors across the country are throwing the book at teenagers who got caught sharing pics they took of themselves with their BFs/GFs/NBFs.) But otherwise, MISSCLEO, I’m going to advise you to back the fuck off. Your son knows you love him, he knows he can talk to you about anything, and he’ll confide in you if and when he’s ready — if, again, this is something he needs to discuss with you at all.