I really think you need to have some major diagnostics done here ASAP.
Could be something going on in brain, tumor, infection.
Could be muscle tear someplace too.
But this is not normal at all.
I will add....is there any chance at all that someone is teasing this horse, when you are not around??? Who cares for him? Is someone else riding him?
If this horse is NOT where you are only one working with him? I would be wondering if someone is taking him out and abusing him.
But get more tests done I think is important.
Also. Who was the vet for this horse before you bought him? He could have had injections or something to manage pain and those are now worn off.
Horses make me a better person.