OP, this is at the heart of it, and to the extent you can encourage your DS to move forward with what seems to be a successful life, you should.
However, it seems that there is something to be resolved in that life. Was there some kind of trauma/hurt in the path he took? Whether it was related to being held back --i.e., bullying about being older, bigger, etc. – or whether he is assuming it would not have happened had he been in the other year, it seems to have left a mark.
To the extent that you as a parent were involved in this decision, it could be affecting your relationship as well, and that too could benefit from exploring and attending to. Does he feel you were/are too controlling? That you prioritized his “doing well” over his friendships and happiness? That you minimized his successes because he had been set up for them? Or simply expected those success because he was older? That to the extent he protested, you didn’t listen? That you owe him an apology?
We DO make decisions for our kids when they are younger, and we make them in their best interest and out of love. If he is complaining to you about these, it may be a signal to you that there is something in your relationship you should talk about and address. Sometimes complaints about old issues are actually expressions of current ones.