Robert Irwin is giving a peek at what's down under. The son of late Crocodile Hunter Steve Irwin leaves very little to the imagination in racy new ads for Bonds, the Australian underwear brand that's launching in the U.S., and he tells PEOPLE that he had no hesitation about his nearly naked modeling moment. "It was a pretty immediate yes," the 21-year-old conservationist and TV personality says with a laugh.