This is sad for all of you; at least try and make sure he has a good, stress-free Christmas.
I was a primary TA / helper for over twenty years. Children don't WANT to be unhappy, but at this age they probably can't analyse what is affecting their emotions.
But I think there must be some root cause behind it. Try and think back as to how he has been during preschool/nursery, and how he then progressed through Reception and the next two years of school.
During all that time, what has he enjoyed and been good at, or what has he NOT liked, been less successful at, and worried about? Somewhere I feel there must be some incident, some accident or clash of personalities - with children, or staff - that worried and unsettled him, and now it has gone on so long that he cannot overcome his fears.
What activities DOES he enjoy and engage in? Does he have friends, both in school and outside? Does he have siblings, and what other family members are around?
What is YOUR attitude to his learning and progress, and how do you react to his progress, his successes or his failures? Could it be that, unknowingly, YOU are setting goals or targets that he feels he cannot reach?
If you clarify some of these points for me, I will try and suggest more specific ways that we might be able to help him.