I'm very glad you didn't prompt him to reject his mother.
I think the appropriate response to your son is, "I understand why you say that, but she is your mother and deserves the chance to be there."
How would you respond if your son were mad at his mother and the two of you were still living together? You wouldn't keep the awards ceremony secret from her.
You and your wife were together for 20 years. She didn't do a bait and switch, she grew and changed.
Me: 44 H: 44 Kids: 20, 16, 16, and 10 Together/Married: 22 years H announced he was emotionally detached and considering D: 4/4/16 H announced he is going to try to stay and reconnect: 5/1/16