Welcome to the forums! This is common PTSD behavior for many sufferers.
Check out this threads:
How Long Do Shut Outs Last?
In summary, there is no blanket rule for how long a shut out lasts.
He gave you a heads up in advance this would happen and his friend has validated this has happened before and I think that's a really good sigh this isn't a response to you personally.
Unless you need to draw a boundary or communicate an end to the relationship or something life and death comes up, I'd suggest backing off a bit. Continuing to text and try to get him to respond is likely inadvertently unintentionally overwhelming to someone who has shut down like this. You are not doing anything wrong at all, it's just probably backfiring a little.
Trauma sends a strong message that love, vulnerability, connection, and closeness are dangerous and postentialy life threatening. That's of course a distorted message, but I takes time and a ton of coping skills and trauma work to overcome it, and for many sufferers, there are times where it can't be overcome.
It's quite common that after a period of closeness it becomes just too much to sustain over time. I have gone on trips and been able to have a great time with people only to shut down for a period of time as soon as we were apart. It wasn't entirely healthy, or about the peoples I was close with, just the best way I knew how to manage everything I was feeling.
His shutting down is probably the best way he knows how to cope with Christmas related triggers.
Try to see this as an opportunity to see if this is the right relationship for you and if/how you can make it through shut outs like this. Connect to any other platonic safe relationships you have and try to use coping skills and good self care to endure the anxiety or grief you might feel.
Also look up advoidant attachment, ambivalent attachment, and preoccupied attachment. It's quite common for ptsd sufferers to have avoidant or ambivalent attachment and to attract people with preoccupied attachment patterns - and it can lead to a bad cycle of the sufferer shutting down and the supporter almost chasing them and the sufferer shutting down more and the supporter chasing even more. This may or may not be happening in your case, and either way, the more secure and steady that you can be the better this might work out.
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