Billy's hair is the classic military cut drawing out the army associations of the underwear colour. The 'cut' of the pants themselves shows off some fine thighs. The loose fit around his buttock in this picture is by no means unappealing.
Billy is forcefully repositioned on the bed. There's a nice, artistic, curving line through his body into the hands of his tormentor.
The twist of his body exposes a selection of highly-touchable, defenceless body areas which we can see in great detail thanks to the colour balancing, the subtle shading to the sides of his abs, the tuck of material between his legs, the hair on his thighs and calves and so on. You can even see what look like bleached areas on the underpants, suggesting a great deal of washing or perhaps it's some sort of dried fluid, in which case let's hope it was Billy's own and not some previous user's.
The crisp briefs detail in this image underlines the point, you may not be excited by the jumble of loose fabric in Billy's crotch but it looks to me like something's trying to get out of the fly opening. The pure white ropes round the chest pinning his arms tight behind make a beautiful finishing touch as his captor contemplates his next move.