Sounds like he has some sort of panty fetish. For the most part, fetishes aren’t anything to worry about, but he needs to learn how to conduct himself so he’s not stealing panties or being creepy or unsafe about it. If his behavior is problematic (I’d say the stealing is sufficiently problematic), consider having him see a therapist who specializes in these issues.
For the record, this has absolutely nothing to do with the ADHD.