It’s a shame that your son is continuing to ruminate on the past, despite his very successful present and likely even more successful future. I agree with the others who have expressed their concern that he get some help sorting through those feelings and learning to let go of things he cannot change. I think you, OP, might also benefit from talking things through with a therapist.
I have a fall birthday and I found it frustrating to be the youngest throughout school. I also had undiagnosed ADHD and didn’t really live up to my potential in high school and a chunk of college. It took me an extra year to graduate, too. But I wouldn’t change it and I certainly don’t blame my parents for making that decision. I was a 4 year old who taught myself to read and my birthday was just before the cut off—hence, I went to kindergarten at 4. I have gone on to have a successful career and have raised 2 lovely kids.
Both of my kids have spring birthdays and I was rather relieved they would be on the older side. But then the cut off date changed to September 1, redshirting became very popular and my March/April babies ended up being on the young side. Oh well. They are fine. And your son is fine, too. The sooner you both realize it, the sooner you can move on and start enjoying his successes!