So... what exactly does he like about girl's panties? The color, shape, texture, design, smell, taste.. what? Does he like to see camel toe, butt cheeks.. hmm? He doesn't fap to it so it's safe to say he's not perverted but once he sees them he just kinda forgot about it and move along. Is it some kinda mental masturbation or something? See, pump, dump, repeat kinda fetish.
Girls do change panties with different designs so why not just get a girlfriend and dress her up. He can buy her millions of different panties so he would practically jizz his brains out every second of the day.