I think the boy's reaction to his dad's hand on his butt is oh so typical of today's kids. A couple of decades ago the kid would have thought it funny, larked about and probably placed his hand on the man's butt! BUT today ... children mustn't be touched anywhere that can be considered remotely 'sexual'.
Your reaction, 'K', was the proper one but I don't think the man was being an 'asshole'. If a dad can't play with his son's bottom, who can?
Jeez, it was just a bit of innocent fun and something I wish I saw more often. The kid needs to get a life and stop listening to all the crap psychobabble put out about kids and sex these days.

Gee I used to do it all the time as a school teacher, especially during swimming lessons, PE lessons, etc. Kids used to regard it as a sign of approval or encouragement. How times have changed, these days teachers are are afraid to be anywhere near a kid unless there are twenty witnesses. Some schools here even ban kids from touching each other. Society is sick.