When my XH and I married we were told my XMIL that we must choose who we were going to have at our big day. I understood - after all, they were divorced and she hated him with a passion. In the end though, we decided that we would not be forced into choosing between them. We invited them both, and their significant others, and let them decide for themselves whether or not to come.
They both realised that our wedding was about us, not them, and they both decided to attend. They behaved as civilly as we could've expected. They each developed the knack of looking into middle distance whenever they were in proximity, and the SO's kept a low profile. It worked so well XH's siblings adopted the same policy; one which is still working to this day.
Your son is not asking you to be buddies with your XW - she is. Her wishes are completely irrelevant. You can make it so that you can still attend big family events whilst remaining civil, yet distant, with your XW. Oh, and if it's any help, from XMIL's experience, she told me it got easier every time. You can do this.
