ML, You know, wifey and S have a situation that is similar to the dynamics of DBing when you have a spouse who wants out.
Your S probably still distrusts X and will carry scars of being abandoned. The fact that wifey did not seem to encourage X to be around S in those crucial years is revealing of her character as well. He needs time to rebuild R w/his dad--not a small task--and not be forced to deal with her, as you know. If she had better emotional intelligence, she would BACK OFF.
The more she pushes/they push, the more he is going to resist. This is especially true because he is a teen. If she left him alone and tried to respect his feelings, there would be a better chance that he would come around. If anything, she is sealing the deal and creating a dynamic where he will never get along with her.
I guess from the perspective of happiness of son (not DBing), it is tricky.
By the way, second marriages often fail because of dynamics exactly like these.
M: 16 years Bomb 4/07 OW 20s long gone Divorced 11/09 I remarried New Guy Cooperative r w/X regarding D