Just my 2 cent's,
Whether he is under guardianship of himself or his parent's doesn't really change anything.
He is a participant of an organized group on a cruise.
Given that he has Autism, and from the OP's report, he obviously doesn't know boundaries and the organizer of the group should have taken action to ensure his behavior was corrected and monitored after it was first reported. The fact he is the organizers son shouldn't be a factor.
The fact that he has been seen wearing a staff t-shirt only makes the responsibility of other other staff to correct and monitor his behavior more important, and then bring's the professionalism of the organization into question given that one of their "staff" has behaved in such a bad way, and his behavior seem's to have not changed.
DCL have definitely dropped the ball in this case as well.
The Security Officers would have handled the situation in they way they thought best on the first occasion, and the situation would have been raised and discussed with the Staff Captain. His opinion on what to do in any further occurrences would have played a part afterwards as well, if not just his, but the Captain.
No-one should feel threatened or scared by another Guest's actions in any circumstance, disability or not, and the situation should have been handled a lot better IMO.
Ex Techie