OP I knew you that your were going to say your DH was forces based on your description of his triggers, I know because my DH has PTSD too, and I know how bloody heartbreaking it can be when they are struggling with it.
You are doing amazingly well OP, and it will get better, my DH had EDMR (I think!) therapy and it took him 8 months to make any progress (as in to finally stop moaning about going and actually see some sense in it!) and the months preceding his turn around were the darkest as everything his mind had suppressed and tucked away for dealing with later finally started coming back to him. He was in therapy for 18 months and he came through in a much better place mentally to cope when he is triggered.
But we are mindful that there are some activities that we just can’t do anymore (mostly involving crowds!) but DH is now 95% of the time having good days and his quality of life has improved massively.
If you think that DH is going to struggle with Xmas then I think you need to take time out from the usual activities, if missing church bothers DH you could find a quiet corner at home to do some prayer/reflection/etc. Could also be worth a phone call to your local pastor for a home visit.
And don’t be afraid to miss the family gatherings, you could have short visits at home with smaller groups if you want to see everyone?
And agree on a safe word so if DH feels like he needs to escape he can do so and you know why and where he has gone.
PTSD can devastate lives, and it’s soul destroying enough just watching your loved one suffer with it, you have all my sympathies 
On a practical note please get in touch with Combat Stress they are a fantastic organisation.
Wishing you and your DH well x