There can be a lot of causes of torticollis in cats, ear infection is high on the list. However, saying that, “bad” ear infections arent that common in cats (compared to dogs) and often have another component like a polyp causing the infection. Once the infection is cleared, the torticollis should go away. However, if there is inflammation of the inner ear, or a polyp (these happen more frequently in cats than dogs) then he may not straighten out his head until these are addressed. If it was a true vestibular event (not related to ear infection) then generally in time it will return to normal.
Best way to diagnose whats going on in that ear is a CT scan of the head. Not cheap, probably run you $500. I dont know if I would do this for my barn cat, but its an option if he is your heart kitty. These images will disect the entire inner ear and show any polyp, infection, malformation etc.
Good luck with him! If he is eating/drinking normally and not vomiting than he is likely handling life well sideways!!