In the firm I work for, we deal with much secret information and many paranoid clients. Generally when someone is terminated we give them a severance contingent on signature of a pretty ironclad non-disparagement letter and a further reinforcement of existing NDA. You could extend this concept to this situation, crafting a letter such that if he <i>does</i> ever leak that data, his liability is assured and his incentives are aligned with yours.<p>This is a time to spend a bit of money and speak to your external counsel. They will have a good solution for you.