If he was in rehab, and lots of people can attest to his using and are willing to say so in court, that is a lot of evidence. The burden of proof in these cases is not the same as in criminal court. The "beyond a reasonable doubt" burden does not exist.
Does his job drug test? If so how is he passing it?
You haven't had him arrested? Why the bleep not? You are only helping him with his problem, not to mention your divorce case.