The bottom line is that even if you could have challenged it 14 years ago, it is too late to do that now. You do not say how old your son is now. If he’s at least age 18, then it is completely up to him what name he wishes to have. He’s free to change the name to anything he wants. You, however, cannot force him to change his name.

If he is not age 18 yet, then he’s at least age 14 and as a practical matter a court isn’t about to force a name change on him that he doesn’t want, even if the court could do so. You said it yourself: “Now, at this point in my son's life, it would be unreasonable to change his name back….” That’s exactly what the judge will think, too.

I understand that for some guys having their son carry on the family name is a big deal to them. But what his name is really doesn’t matter beyond a sense of pride in the family name. It doesn’t change who he is or what kind of person he’ll grow up to be. Moreover, there is no guarantee that he’ll have son to carry on the family name anyway. William Shakespeare said it nicely in his play Romeo and Juliet, Act II, Scene II:
“What’s in a name? that which we call a rose
By any other name would smell as sweet;
So Romeo would, were he not Romeo call’d,
Retain that dear perfection which he owes
Without that title.”

It is not the name that makes the man. It is his character that makes the man.