No idea where to go with this....

Given what you have said I feel comfortable to rule out pain as a likely cause.
One thing you don't mention is how often he is worked, what type of work and for how long.
Have you noticed any relationship between working hard and being calmer or more agitated the next day?
Work has a massive (disproportionate) effect of some horses. I would be exploring this route keeping a an accurate log of work vs behaviour.
 
Given you have said you have owned him since a foal, what age was that and how was he raised?

A lot of people say homebreds can be awful as they get away with everything and so become very temperamental. I just wonder if it is something like he doesn't know how to be a horse, it certainly sounds like when he is with you he is better so might be a case of temper tantrums to get his own way. What is he like for someone he doesn't know?
 
I have just retired my boy with neck arthritis. His behaviour became unpredictable as he was in pain due to his nerves being impacted and exhibiting neurological symptoms. I wouldn’t think that would be the case with yours if he’s behaving riding. Have vets suggested Bute trials to see if behaviour improves?

My boy behaved totally when ridden. He was diagnosed with neck arthritis 2 weeks ago, and was PTS yesterday. he was diagnosed with severe changes, and 4 different vets recommended PTS straight away. Some horses just seem to cope better with pain.

OP - I would have his neck xrayed again, he sounds very unhappy. You say he throws his head around after eating, the neck position he is in to eat could easily be causing him pain.

Hope you get him sorted
 
I had a horse with those habits. Particularly the head twirling (dreadful) and panicky behaviour in the stable. I'm sorry to say that in his case it was due to a brain lesion. We had no option but to pts, as he was becoming a danger to himself and to us. Heartbreaking, it really was. I would suggest that your vet do some tests. In our case the horse was shown to be short of potassium, but his brain said he had too much so he was weeing for Britain at the end to pass out what he didn't have. I do hope it's not, but couldn't pass this by. Good luck.
 
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