Is he threatening to post false information? Or, is he threatening to post his account of his experiences with your company.
Your state will have a definition for the crime of blackmail or extortion somewhere within the codes. Since this definition might vary by state, it could help if you mentioned what state this was in.
In the short run, you could consider speaking to the police or even contacting an attorney who can advise you of any possible criminal or civil remedy.
- Carl