IF something that is said directly impacts his job search then yes, you can at that time sue him. But you will want to be very careful that you are right; don't just guess that it MUST be because of something this guy is saying. It's a very tight job market out there; it's taking people much longer to find jobs than it usually would, and a lot of people are ASSUMING that if they aren't finding a job, it MUST be because of something a former employer is saying about them when that is not the case at all. If you file a lawsuit on this guy and it turns out that he was NOT at fault, you'll be the ones in trouble.
While there is no way in this kind of forum to give you a guarantee, here is what to look for:
1.) If your husband is not being called for interviews, I will guarantee you that the former employer has nothing to do with it. Hiring managers DO NOT contact former employers prior to deciding whom to interview. It DOES NOT HAPPEN. If he is sending out resumes and not getting any response, he can look to the market or he can have his resume rewritten, but it will NOT be because of what anyone is saying about him. I am emphasizing this because I have responded to one too many posts from people who are utterly convinced that the only reason they are not getting interviews is due to a former employer, and I have been recruiting and hiring for over 15 years and KNOW that this is not the way it works.
2.) If he is called for an interview, has the interview and hears nothing further, it is POSSIBLE that ONE, maybe two of these occasions has to do with references, but it is quite unlikely. Since the market is tight, the likelihood is that employers are seeing six or ten or a dozen people or more, only a couple of whom will have reference checks done on them (probably not without being told that this will happen) and only one of whom will ultimately be hired. The other five or nine or eleven or more will not, and the large majority of them won't even have been the subject of a reference check. The odds are that this also has nothing to do with the former employer.
3.) If, however, he goes for the interview, is given every reason to believe that he has the job, and then it goes somewhere else at the last minute, once or twice may be because he's the top applicant that DIDN'T get hired (it's rare that reference checks are only done on one applicant in my experience) but if it happens more often than that, you've probably got something to worry about. In that case go to an attorney. Go directly to an attorney. Do not pass Go, do not collect $200.