@tibbleinparadise,
It is common for non-students to attend and I have never heard of a prom where you couldn't invite whomever you wanted. Many kids do not date kids from their school so it would seem odd to me that would be just kids from the school that can attend.
I doubt it is the childhood thing as he was bragging to another boy to leave his prom date alone. He seemed proud that she was going to be his date for prom.
I do believe him simply because I know his parents. To be honest we were not surprised that the parents said he couldn't go. We were actually surprised that they allowed him to go in the first place.
Although I could see his parents using an excuse like his grades went down to say he couldn't go.
We have also known the family since my daughter was in kindergarten. They do not allow their children to do anything that is outside the church or school - but only for school academics, band, or sport (and then only a sport that would not require to be there on a Sunday).
Their oldest child held an alternative prom at the church since she wasn't able to go to hers. The alternative was no dates-everyone came as single, you could dress up or down, dinner was cooked (by the mom), there was not any dancing - there was a variety of games and the dinner did look awesome. The kids did have fun.