Ok... I have NO advice for you... And I'm a guy... So go ahead and yell at me for posting something here if you'd like, but there is a consideration that I'd like to share with you that you may not have taken into account.
It's not just the panties that are the source of arousal for him. It's the fact that a flesh and blood, living woman had worn them against her skin... a womans skin that is located in her most intimate of places...
I'm not sure that merely buying new panties at the store would hold the same fascination and potential for arousal for him... since they've never been worn...
Just a thought... from a guys perspective.
Dan