You've mentioned that you've read the server rules
yourself; but can I get you to confirm that you've also gone over the rules with your son, and that he intends to follow them. We've had a few new players misunderstand the section on griefing lately, so we're being a little more cautious. If they didn't build it, they shouldn't be adding or removing anything near other projects, or from their chests, without direct permission from their builders.
Sorry for the inconvenience, but it'll be a quick process after we get your confirmation... promise.