Surely, you have some records. Someone paid for that domain name. If you can show that the business paid for the domain and that it was his job to register the domain name, but he put it in his name instead of the business owner's name, then I'd think it'd be a simple case for small claims court. (IANAL)
For what it's worth though, I'm familiar with an organization that found itself faced with some of the members of the board of directors being dissatisfied with the direction the organization was heading in. They broke off as a splinter group, but still use the organization's name and pose as the official organization. And, the remaining board members claim that THEY'RE the official organization as well. However, both groups claim the right to the same name - the splinter group had control of their domain name. It's still a big fight. And, it's resulted in the loss of revenue to both sides while everyone sits back and waits for one side or the other to be victorious. (But, personally, it's saved me a few hundred dollars by sitting back and waiting.)
edit: I thought about it a bit, if you can prove that the business did, in fact, pay for the registration, then perhaps you can use this to threaten him with legal action. I can't imagine any reason for someone to sit back and think, "yeah, right, what are you gonna due" when simply turning over the information is simple. At least, not if you get some help from a lawyer to draft a letter to him and send it via certified mail, threatening suing him for damages as well.
However, if you can't proved that your business paid for the domain, it's going to be a tougher battle (I think.)
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