I would be concerned that he himself can be sensitive yet he is blunt with others, and disruptive in class though presumably he has been told that his behaviour has an impact on others in class.
What sort of interventions have been tried to deal with the class clown thing?
Have you been concerned about the 'class clown' description over many years? The 'class clown' is often expressing something by means of inappropriate behaviour - anxiety or insecurity - and this behaviour can be a sign of a deficit in taking on board social cues.
'Bluntness' with peers and class clown behaviour have in common a lack of concern for the impact of words and behaviour on others. There is taking a joke too far here, and the lack of a filter.
Is he one of the oldest in his class just because of a cutoff date / early Autumn birthday or was he held back in preschool or one of the early years?
You say he is NT - has the question of attention deficit or ASD come up before and has that been assessed?
Why is he calling himself 'dumb'?
Where is the thought that he is 'worthless' coming from?
Does he feel or have an inkling that he has a low level of social competence?
What does he tend to sulk about? How long do sulks last? What are the features of his sulking behaviour? How does a sulk end?
I would ask his teacher for a conversation, if I were you, and I would consider asking for professional help - persistent class clown behaviour has to be addressed. It attracts negative behaviour from teachers, and a reputation for that will precede him and cause him problems.
He may well be trying to find a footing among his peers by persisting in class clown behaviour. It's possible that in his early school career this behaviour afforded him a certain amount of popularity among his classmates and was merely seen as playfulness that didn't necessarily attract too much negative attention from teachers. In the early years of formal education, playful boys tend to be viewed positively by their peers but by age 9-10 teachers tend to make it clear that such behaviour is viewed negatively, and the kid who is a law abiding citizen of the school can edge out the playful kid in the popularity stakes. Girls really shine in the classroom in later childhood for this reason. By age 10 the puzzled class clown can't understand why the atmosphere in the classroom is turning frostier by the month - he may notice that the social landscape is changing but he can't figure out his place in it. Meanwhile, he has friends, and the behaviour hasn't alienated them yet, so he hasn't experienced the full consequence of not changing his tack to suit the shifting wind. He is aware of undercurrents though.
It sounds to me as if your DS has got a little 'stuck' in his approach to the social environment, and could use some help. I would be concerned that he is quite an anxious child underneath it all, maybe insecure, and hasn't gone beyond his behavioural comfort zone, or grown in his response to frustrations. It would be a concern for me that he sulks. Bluntness not so much in and of itself as long as it wasn't hurtful. But sulking and being sensitive when he can 'dish it' to others (not co-operating, maybe hampering a game in doing so) and seemingly not notice the effect he has is a concern. Certainly in tandem with continued class clown behaviour, the lack of concern for social cues would bother me.
The devotion that was well above normal levels to the Santa business would also be a concern. Cognitively, he should have figured out that there would be practical difficulties associated with a Christmas Eve circumnavigation of the globe using flying reindeer, and a sleigh full of pressies sufficing for billions of children, some of whom do not have chimneys. From an emotional standpoint, he was writing daily letters that were never answered. What was he getting out of this experience?
I wonder does he see others as real, three dimensional people? Is it hard for him to put himself in others' shoes? Can he accept someone else's perspective?
I would be concerned that he is a bit rigid in his responses to people and situations. He doesn't seem to adapt to fit the needs of his environment or people in it (keeping on plugging away at the letters regardless of lack of feedback and also regardless of what observed reality must have been telling him about the Santa setup is an example of not connecting dots).
A good many children believe in SC to age 10 and even beyond. But daily letters to the elves takes this to another level entirely.