Most people’s executive functions (part of brain functioning) are fully developed in their early 20’s. For those with ADHD (a developmental disorder), this doesn’t fully mature until approximately 28-30. Executive functioning is planning, organization, task initiation, transitioning from one task to another, emotional regulation, time management, etc.
The bottom line is that your current way of communicating isn’t working for either of you, so you need to try something else. The idea of writing down requests is helpful. Definitely put them where he’ll see them. And schedule family meetings instead of asking him to stop what he’s doing in an instant.
The natural consequence in this situation is that he doesn’t have input on the vacation, not that you leave him out of a family event (that sends the message that he’s not loved or wanted).
I challenge you to change the language you use to describe his behavior as well. Instead of thinking of him as “insensitive and rude,” shift your thinking. “He’s having a hard time managing his frustration right now.” or “He doesn’t mean to be rude, he doesn’t see that his behavior hurts me.” Etc…
ADDitude Community Moderator, Parenting ADHD Trainer & Author, Mom to teen w/ ADHD, LDs, and autism