OP - I think you ABU. I lost my Mum very suddenly last year, and would have cancelled Christmas completely if I had the chance. This year is not much better.
I also have no family left really, and it is difficult seeing other people with the parents and family they still have. It really rubs in just what you have lost.
He must feel all that too, and will be quietly coping with a lot of really awful feelings of loss, regret, memories and so on. It really is so hard, and I’m not sure people who still have their parents/ family can understand what it feels like.
I say let him go, with your blessing. Explain to your other family members why and where he is going and ask them for help with the DC’s/ cooking if you need it. Or move the meal time maybe.
Let him have that little breather and chance to think about his Mum. He needs an outlet to do that. I took an hour out last Christmas Day to think and cry about my Mum. I just turned the oven off and buggered off
not great, but I needed it, and after I had a good cry and think I was ready to get back with it and focus on my DC’s as the family I still have, the future and not the past.
Let him have his time - the last thing he needs is to be given grief about going for a breather. Hopefully next year he’ll be coming to terms with if more, but for now, whilst it still must be so raw, I’d leave him be - it’s just kinder.