I am very sorry for Remy and you that this is happening. Sounds like something is going on with his back, or something neurological, or a combo. Is there any possibility he injured himself?
A more remote possibility is AAI, although I hope it is a good sign that Remy's xrays appear to be normal (are they going to be read by a radiologist?):
https://www.acvs.org/small-animal/at...al-instability
I believe there is one more neck/spinal thing that Yorkies can have, although still relatively rare. I'm sure other members will have more info/suggestions.
I believe it would be a good idea to take Remy to a neurologist as soon as possible. I would keep him crated in the meantime.
Please keep us updated.