I'm glad there was no fall involved. But I'm sure that figuring out the next steps are difficult for all involved. What treatment has your dentist recommended? Medicine, exercises, supplements?
Here's a page of treatment suggestions for temporomandibular disorders that may help:
And this page also had some holistic suggestions:
Does your dentist specialize in TMJ? Some do. Have you asked your son's ataxia doctor about the TMJ?