If this is going to be a spanking.....

.......why's his father checking out his hole, if that's what he's doing? Maybe seeing that he's keeping it clean, and if not giving him extra strokes?
If that was me naked over this guy's lap feeling my dangling cock brushing up against his khaki shorts would be sufficient alone to give me a first-class boner - but that's just me finding it such a turn-on, one of the lifelong legacies from my experience in the Boy Scouts. So anyway, once the slapping started it'd be sure as shit that I wouldn't be able to hold back from cumming, no doubt earning me more strokes for dirtying his shorts.