Is your husband taking any meds ? Some meds cause bruxism (grinding). How is his neck doing ? Has his apnea been diagnosed as obstructive and/or central ? Central sleep apnea often relates to inflammation and injury in the upper neck/brain stem.

I don't remember if it has been mentioned in the past. There is an imaging system that looks at the stability of the upper neck with a moving image as he moves his head. It would be worth some local research. Has he been evaluated at a neuro/spine rehab clinic ?