It is true that opinion is not actionable as defamation. However, it appears that several of the statements that he made are factual. If they are false and he knows (or should know) them to be false and they tend to injure you in your occupation, then you may have an actional claim for defamation. You may be able to sue the client in California based on whether he has "minimum contacts" with California. You should consult an attorney in this matter as there are several issues that need to be analyzed in order to determine whether to move forward with it.
I would add that while you seem to frame your question as whether you individually have been wronged, it seems from the facts you've given that your legal recourse is to pursue an action for business disparagement, which is a form of defamation. In such a case, your concern is the loss of current and/or future business due to the disparaging statements the individual has made to third parties through this site. Seek the advice of a local attorney, many of whom undoubtedly offer free consultations.