Hi Brother,
I am torn as well by the decision that you need to make. I think if I was in this situation, I would try to find the most God-Honoring and most loving course of action to take.
I agree with you to not celebrate or endorse the sin of another, and if possible to not let your son attend the wedding either. I would first spend much time to pray that God continues to hold our marriage together despite the already difficult circumstances. I would then try to talk to my wife first about this situation and what my convictions are. I would then talk to people who can support me spiritually and pray for me too.
One last practical advice I could suggest, if your wife insists on taking your son, then pray that your son will ask the question, why did my Daddy not attend the wedding with me. At least you can still set an example for him through your own actions.
Will pray for you as well.