Perhaps a pic of him wearing the panties is in order :3 You know, just for your personal album
Since you found another girl's panties, I have an idea that might make him keep it in his pants.
Get him some underwear that says "Property of Princess Diana," he won't get too handsy with other girls with your name on his drawers! (For that, check out
https://www.customizedgirl.com/, they have options to customize lots of things, including men and women's underwear. You could include your name or even your face on them.)