Congratulations to your son! I was one who posted a number of times on your other thread (our son was a a fall birthday and we did hold him out of K for a year). So our son was one of the oldest in his class.
As I said in my other posts…a good life skill for your son is to acknowledge his accomplishments and not compare himself to anyone else. As he grows and matures even more, and grows into his adult status, hopefully he will see that what he has accomplished is more important than how long it took him to get there.
It sounds like he has much to celebrate…and I hope he will get there soon enough.