Monday, Aug. 5, was National Underwear Day, and Riley Green didn’t let it slip by without a celebratory post. The singer is a brand ambassador for Gildan Underwear and shared a collection of promotional photos that had fans in the comments section, well, audibly drooling. “Happy National Underwear Day from me, Carl, and Gildan,” the singer writes. The main photo is innocent enough: Green is wearing a grey T-shirt and jeans and holding his beloved dog, Carl. But scroll through the carousel and you’ll see the pictures that got everybody talking. In a second snap, the “Rather Be” singer is pullin…