I can understand this is a frustrating situation. No cause, fault, or any reason need be given to terminate an at-will employee. An at-will employee can be terminated simply because the employer doesn't like the employee anymore, wants to hire new people, or just because they change their minds. Illegal termination is termination on the basis of race, gender, religion or other "protected status." However, employees with a contract that states they can only be fired for "cause" (and sometimes those contracts define what "cause" means), can not be terminated at-will and a legitimate reason must usually be given. Even then, the employer usually has some discretion in determining what is a legitimate reason to terminate and the employee doesn't necessarily have to agree that the reason is legitimate.
If your husband had a written employment contract indicating he was not an at-will employee, you can certainly try to speak to an attorney regarding whether there are any legal options. You can fin done through the locator service on LawInfo's home page at:
http://www.lawinfo.com/.