Bizarre. This would make me want to look into the home life of this child. I'm trying hard not to make assumptions, but it is so strange.
Kind of makes one wonder if one or both of the parents are into pornography.
The boy has either been abused himself, or (more likely) has easy access to pornography at home, either on cable or on the Internet.
Unfortunately stuff like this is happening ever more frequently at school. At my local primary school a young teacher was asked on the first day of school what color her panties were (this was asked by a 7 year old boy).