Frizzle can occur in many breeds but is more common in certain breeds like bantam cochin. If you cross a frizzle Cochin with a silkie you can end up with a sizzle. To tell if your bird has silkie in its breeding look for black skin, extra toes and usually a small crest.
The picture is blurry so it's hard to see these traits. Silkies can have domed skulls that makes them vulnerable to head injuries from pecks, some also can have open areas on their skull even if they aren't domed.
I once had a domed skull silkie that did get pecked in her skull and was never right. Very dumpy and slow, and prone to falling over. She's the main reason I don't keep silkies anymore.
The other option I was thinking was Mareks. It's possible it's advanced far enough to cause the shaking. We generally cull after they can no longer walk efficiently. I doubt your little bird will improve unfortunately, but definitely try vitamins because you never know.
