Hello again and thank you for your patience. You always present such a common sense approach to situations and intelligent deductions. Unfortunately, the 'human factor' is a wild card that doesn't follow common sense or, I'm afraid, intelligence. This you know and it's being proven with this 21 year old adult-child.
You need to take a moment to reflect at how sure you were about oh so many things when you were 21. How incredibly smart you thought you were and how you knew the answers to questions no one ever asked. Why, at 21 didn't we all know the simple, absolutely workable solution to pretty much all of the worlds problems? Oh the things we didn't know we didn't know.
Like this adult-child had to fall a lot before he figured out how those chubby legs worked; like he used the wrong words and cute mispronunciations to communicate before he learned how to really talk; like he had to suffer through the slings and arrows his peers threw for whatever reasons and he surely thought his life as he knew it was over and yet, there he is, out the door without a look back and seemingly without regard to the same people who got him up on those chubby legs, taught him how to communicate and supported him through the tough times.
And you have to let him go just like you have gone through your own life. We didn't choose to be here; we had parents who made the choice to bring us here. So it's they who owe us a living, so to speak, until such time that we are old enough to be on our own. Families have to trust that they raised their children right and even though there's a period of seeming ingratitude or disregard, all we can do is wait (for them to come to their senses?).
Sometimes it may never happen, for whatever reason(s), but that's not the norm. Let him work out whatever he needs to work out. Let him age a little more and learn that he's not as smart as he thinks he is. Some of us come to that realization sooner than others; some - never.
In the meantime, don't push or pull. Send a card on holidays; make a phone call on special occasions to
share a story and always make it clear he's missed, would be so welcome at any event or even just to drop in for no reason.
Trust not so much in him right now, but in yourselves.
So the bot***** *****ne is that you're right to feel how you feel and even more so his parents; however, he can't be forced to fit a mold that right now he's clearly making a break from. Carry on as usual, resist the urge to call him out on the carpet and just wait.
It's really all you can do.