Unjustified demotion

My husband was given a promotion into a managerial position. Recently, he was demoted, though his supervisor will not call it a demotion. All of his managerial responsibilities have been removed and he is essentially back where he started. The supervisor refuses to tell him what exactly his new position is and responds evasively to questions regarding what his job title and description now entail. His name has been omitted completely from the new organizational chart for his department. He has never been approached regarding any issues regarding a lack of performance. His performance reviews have been exemplary throughout his 6 years with this company, though he was never given a performance review last year despite company policy requiring a yearly review. Three days prior to this reduction of job responsibilites, his manager threatened him in an attempt to get my husband to issue a statement absolving the manager of misconduct in another matter. My husband would not write the statement because the facts pointed directly to misconduct on the manager's part.
It is apparent that the manager is attempting to use a general "sub-standard performance" excuse to justify the demotion. Does my husband have any ground to stand on? He feels as if there is really nothing he can do about the matter. He also feels this is the first step the manager is taking in attempting to have him terminated. He has approached the H.R. Director, but has not received a response. All his (former) reports support his managerial performance completely.