Can an attorney threaten you by reporting you to the IRS?

That would seem to be a violation of the attorney client privilege. I suggest you tell him that you are thinking of reporting him to the bar.

I am not sure why this is under the Landlord/Tenant section, but either way, it seems more information would be needed to properly address this question. Generally, an attorney is prohibited from disclosing confidential information about a client to third parties. There are exceptions where an attorney may have a legal obligation to discuss in specific narrow situations, such as concern that the client is a danger to self/others or has indicated he/she will commit a crime, which some IRS violations are in fact criminal. So it would be helpful to know whether the matter your attorney is currently representing you on has to do with the IRS matter in question, whether there is already an IRS investigation of you going on or this would be a new "tip" from the attorney to the IRS, whether there is any truth to what the attorney is claiming you have engaged in, etc. You should consult with an attorney (a different one!) to discuss further, as well as review the text messages.