As the WAW, I think it's horrible that your wife is putting S4 in the middle. I would call so I could speak to my son and tell him that you have plans but you love him. Tell him that the 2 of you can do something special another day. Don't engage the W if at all possible. I get that she may answer but I would just keep saying you need to talk to you son since HE called you.
As you know, I'm still new at this but if it's what I would do.
Doodi
"I feel like Rocky in the fifteenth round, beaten beyond recognition, when I realize, the only person I've been fighting for is...myself."