We all agree that the MRI is important here. It sounds like a nerve or neurological issue.
Thank you for your additional information, it helps.
Rimadyl is like Advil... so you have an anti inflamatory / pain reliever going right now.
House Diagnosis: It sounds like when he slipped on the floor 6 months ago there may have been a more definative issue. He could have slipped a disc or had a hernyated disc. In the later the fluid in between the discs gets pushed out of the capsule (like a jelly donut). In a human this is very painful because our spine is vertical and we are contantly compressing this space. In a dog there may be less pain (don't know for sure, everytime I ask them they don't answer... :)) But if this fluid has compressed other areas including nerves, blood flow...
The condition you describe (Fibrocartilaginous Embolis) is serious and has no injury basis associated with it. Don't want to go there yet.
It is possible that the Rimadyl has helped with the pain/imflamation for this amount of time but the injury has progressed to a new symptom?
MRI is the way to go for sure.
Does he swim? Do you have a pool or lake access?
We wonder if you took him in and held him as he floated/swam what you would see with the asymmetric paralysis? Would it open up the space in his neck and back and could you see improvment? Just more of an experiment then a diagnosis.
Don't get yourself worked up just yet, it could be so simple to fix... Contact me again and let me know what the MRI said. We are all interested in the outcome.