With his fall/accident, I wouldn’t be all that surprised to see a fracture. He might have just really wrenched it (in which case, Xray could look good but maybe ultrasound would pick up inflammation). Might have just aggravated things with some preexisting arthritis.
Since it’s been a little while, it sounds like it’s maybe too late to do DMSO IV? I’m not really sure. Other possible treatments include keeping up with the steroids, using different steroids (such as injecting joints in the neck), other NSAIDs, acupuncture, laser, PEMF (probably NOT chiro at this stage).
I’d want to know what was going on in his body, whether it is the neck or the back or his pelvis. And then treat that medically as best as you can. Then based on that information, move on to some proprioceptive training. Lots of ways to work on this if he can at least walk around ok on controlled footing. Using things like the Equicore bands, using the T-Touch figure 8 wrap or something like that, putting a cat collar on a fetlock to increase awareness. Hand walking very carefully over changing surfaces (nothing crazy here but maybe like from road to grass or sand). Maybe backing up a couple steps.
I do think it’s unsettling for them to suddenly not be in control of their bodies like they used to.