The thing is, there’s a certain mindset involved in fixating on how a different path in life would have been better, and it’s a mindset better challenged and deconstructed than indulged. How can you (or he) know that if he’d finished high school a year earlier, he wouldn’t have been equally bitter about the downsides of that choice?
Respectfully, I hope he’s getting some help with gaining perspective. Achieving the level of success that he has and still dwelling on this water-under-the-bridge issue seems like either 1) he’s deeply “stuck” on something that is the very definition of a first-world problem, or 2) he’s getting something out of continuing to guilt-trip the caring parents who tried in good faith to make the best choice for him, with the information they had at the time.
If you’re continuing to apologize for this (either overtly or implicitly), I would suggest that you let him know that you are letting yourselves off the hook and won’t continue to engage or self-flagellate on this issue. He has a great life, in no small part because his parents helped to prepare him for it. Refocusing on gratitude could go a long way.
ETA: Sorry, I misread - the choice was not to redshirt him. Either way, though… he’s long overdue to stop wishing for a different past. It’s one thing to dispassionately believe that the other choice would have been better, and even to help communicate that perspective to parents who face the same choice today. It’s another thing to let his present and future be clouded by being “haunted” by what might have been. I hope he can put this behind him and move on. BTW, I got grade-skipped from 1st to second and started college way too young, so I definitely know what he’s saying. But I also know that the road not taken would have had downsides too.