I am so upset, FIL has refused to go with our xmas plans this year and now I don't know what to do.

I can absolutely sympathise with how heartbreaking it is to be in the situation you are in with your mum.

However, as far as your FIL is concerned, it has to be his choice and everyone else needs to respect his decision.
It doesn't sound like he is expecting anyone to rearrange their plans.
It is likely he just wants time on his own.

This is an old man who spent 25 years going to your house on Christmas day. You did extremely well. I did the same type of hosting for many years but for both sets of parents etc so I know what it's like. It was hard work but I did enjoy it.

It's understandable given your mum's situation you want to change your plans. If your fil was a young man with his life ahead of him I would describe him as you said. As it is he is still grieving the loss of his wife & taking comfort in his memories. In all honesty OP I think it's very unfair to describe him the way your doing rather than trying to understand it from his stage of life & what he has lost. It's stressful with your mum having alzheimers also when parents become less capable of usual routines such as Chrismas. They should still be respected for what they are & what they are now.

I would do what a pp suggested. I'd go along & have a chat with him & explain the situation & your predicament then see his response & take it from there. If he still refuses your DH could visit him on Christmas morning,make sure he has food to eat & leave him to settle for the day in his own environment.

Can he come to yours for 6pm even if he doesn’t want o join you for lunch?

No idea why you are so upset, you don't like your FIL and now you don't have to spend xmas Day with him.
Of course the issue is whether your DH will choose to and I would be pissed off if that were the case in your shoes.

You specify to FIL that the invitation remains open and carry on with your original plans.

Spending Christmas apart from your DH is just bonkers, why would you do that?

Leave him to it.
Leave your dh organise the whole Christmas thing in his own for his dad. Do the running around etc…

THEY are BOTH choosing the situation.
And they are both doing so in the knowledge of the reasons why the organisation has changed this year.

Yes it’s is stubborn old men. Because they are miles too used to everyone bending over themselves to please them. You included.

I think I'd do this too, if he really won't come.

@Dreamsfallapartattheseams
You say 'There is no persuading FIL, he is as stubborn as a mule. What can we do?'.

You can respect his decision, as an adult in his right mind, to spend Christmas Day on his own.

My late mother often chose to spend a quiet Christmas Day on her own. Sometimes she missed my dad so much she couldn't face an afternoon of faking jokes and jollity with my family and various in-laws and outliers while her heart was breaking. She would visit Boxing Day when things were quieter.

Me and/or adult DC would, go over early in the morning or on Christmas Eve with a basket full of treats (turkey sandwiches, mince pies, crisps, wine, chocolate - all things she wouldn't buy herself) and presents. We'd give her a hug and a kiss and then leave her in peace.

Your FIL is old not stupid. Let him know there will be a space at the table if he changes his mind at the last minute. Then respect his ability to choose what's right for him.

Honestly this reads as though you are trying to organise your FIL to do what you want in order to preserve your Christmas while your mum can still appreciate it. I understand that but it’s unfair to expect him to go along with this.

You say you cannot leave him on his own on Xmas day, well of course you can. His choice. See him another day. Maybe he has a friend he wants to meet up with on Xmas day ?

I would tell him that you'd really like him to come but if he really doesn't want to that's up to him. Sometimes it's just not possible to accommodate the wishes of everyone. He's made a choice of his own free will.

Surely it shouldn't have been you inviting him, your dad / mum should have been inviting him if you are hosting Xmas at their house?

If I've got that right then I would let your parents invite him a bit more formally. That said I wouldn't like this idea too much if I were him. It's one thing having Xmas at your child's house, but going to your child's, husbands, parents house may be a step too far unless they are very close.

Out of interest have your parents agreed to all of this?

I’d respect his wishes. He might be ill or in pain or just find everyone a bit much. Plate him a dinner, pop round for a coffee and do presents and then off to mums. Simples.

Can your father call your FIL to invite him? Perhaps your FIL feels that you can only be invited to someone house by someone who lives there? I assume FIL has been told the reasons for the change of plan?

If that won't work, then maybe your FIL can come to your house in the evening, or you and DH go to his. Either way, I don't think you need to feel guilty about him being on his own when you've made reasonable attempts to include him.

This is absolutely what you should do imo

This is why it's not good to have rigid Christmas traditions and arrangements, someone gets upset when they inevitably have to change.
If you start mixing it up when everyone is young and healthy, it's much better in the long run.

Anyway in this case leave your FIL be and don't guilt trip your DH with the whole "first Christmas apart in 25 years" bollocks in case he would actually prefer to spend time with his elderly father.

If he is close enough I would offer for someone to take him a plate when it is convenient but apart from that go ahead with your plans, there is absolutely no reason to change them.

As others have said, you invited him, he has said no, he can decide what to do on Christmas day once your Dh tells him okay, that is your decision.

Stop pandering to this man and spend quality time with your Mum who may sadly decline further and this might be the last good Christmas you get.

Leave him, it's his choice.

Can you not have lunch at your parents a bit later? So your husband can eat with his dad at 12, then come to your parents for a second lunch.

Or leave your fil.aline for lunch but do the buffet in the evening like you usually do and he comes to that.

He had an invite and declined it. Your DH does not have to go around there. If he does then it could set a precedent, whereas if FIL spends the day on their own then it will help decide what to do for future years.

FIL has made his decision and you all need to respect it. We had the same with XMIL as we never wanted to leave her on her own, but one year she was absolutely adament that she was not going to go anywhere, so we left her to it.

It's their life , their choice. I get that you/DH feel bad, but he is entitled to his decision.

You and DH need to continue your plans as usual, but maybe plan to call in on him at some point, maybe on the way home with a plated dinner for him?

Your FIL has declined the invitation. Don't change your plans. Your husband takes a plated up meal for his Dad on Christmas Day. Your husband then comes back to you. Don't start sending your husband to his Dad's for Christmas lunch...your FIL will begin to expect that every year! Your FIL has made his choice, respect it, and carry on with your plans.

There's nothing worse than people making plans, and arrangements then expecting everyone to fall into those arrangements. Even worse is then putting pressure onto people to comply with those arrangements.

You've invited your FIL, he's declined. It's unfair to then put pressure on him to comply with your plans. He knows declining means he'll be alone... He should have his decision respected. Just get on with your plans, and leave him with his.

This!
And just say, the invite remains open even if you change your mind on the day, we’ll have more than plenty enough food to go around.

He’s a grown man who can make his own decisions even if you disagree with them.
Let him stay on his own if that’s what he wants.

Don’t let him win with the emotional blackmail.

Where's the emotional blackmail? He's not demanding they change the arrangements, or complaining about being alone. It seems to be the op who wants to pressure him to change his mind and do something he doesn't want to do.