Tell your ex he is not spending Christmas Day at your house or your fathers house.
Ask him if he wants DS on Christmas even or Boxing Day.
Once you have agreed which day DS is going to see his dad, tell DS.
“ As you know dad has left home because he has a GF so he and I will be getting a divorce. This means we won’t be having Christmases or holidays together anymore because we are not a couple / married. So this year you are going to spend X day with dad at his new home and Christmas Day with GD and me at GDs house.
“ I’m sorry if this upsets you. I know that you would rather this hadn’t happened and we were all still together . But dad is an adult and I can’t stop him leaving and being with who he wants to be. I feel sad too but that’s his choice . It was nothing to do with you or anything you have done - he loves you and he will always be your dad. “.
You must know that your ex is feeding your son these lines. No 10 year old uses the phrase “ you should be with your husband at Christmas “.
In an ideal world your husband would stop using his child to get his own way. But nothing you say to him will stop him. So you can only deal with DS and be very clear with him. All these things - Christmas , holidays, birthdays apart are what happens when a couple split up.
“ It’s confusing and hurtful for everyone ( really you and DS) to have to pretend that nothing has happened. We all have to deal with the reality of the choices that dad has made “.
He needs clarity so he’s not manipulated into this same situation time after time. So when his dad says “ oh I’d so like to spen Christmas Day with my lovely wife and son at my FILs” your son needs to know that your husband is mis representing the facts
- you are no longer his wife because he left you
- most separated / divorced couples don’t spend these days together, as it’s too hurtful, especially for the person who was left. Maybe in years to come , when/ if you both have new partners etc. But not now when it’s too raw.
I’m afraid you are going to have to be a lot more honest with your son. Ideally he would be kept out of it all. But since your ex is so manipulative, you need to equip your son with the facts.
It’s not your job to protect your ex from the consequences of his own decisions.