I am preparing to resign from my position at a private engineering firm I’ve been with for 5 years. My boss and I were both previously employed by the federal government at the EPA (he was my boss at the EPA as well). He left that job to start his own engineering company and recruited me to join his team. My position at the firm is to provide all EPA-related guidance and ensure we remain in compliance with governmental regulations. Since my company’s projects are monitored by the EPA, I still maintain a professional relationship with my old coworkers and must correspond with them on a regular basis.
One month ago, a former colleague/friend at the EPA encouraged me to apply for a vacant position at the agency. She also said that the person who is currently in the position my boss used to hold would like me back at the agency. I applied for the position and accepted an offer of employment last week.
I know my boss will find out, no matter what, that I'm returning to my old company, but I would rather him learn the news from me than someone else. I am also anticipating major shock and anger from him. He has created a very hostile workplace culture over the past few years and the last three people who resigned didn’t get to stay their two weeks. My boss immediately escorted them out of the building, even though their resignations (in writing and in person) were extremely professional and respectful.
Because my boss is a very powerful/wealthy individual and does so much work with the EPA, I am worried when he finds out he will call someone important at agency HQ in Washington and try to sabotage my new job before it even starts. Once I’m back at the EPA in a few weeks, there will be nothing he can do to me; however, until my employment there officially starts, I am concerned of what he will say or do.
My question is this: If I don't mention I'm returning to my old company in my resignation letter to my boss (and HR), what might the consequences be? What are some possible strategies for communicating this fact to my boss?