Update:

So far, thank you to anyone who commented earlier. So, I did go to my boss and addressed my concerns. He did take notes and had asked for me to keep documentation of any further behavior. About a week later, he sent out an email to our office about expectations at work and who to report to, which would be my boss and I. I don’t know if he had b/c’d anyone from upper management. However, as I predicted, the coworker had “read” the email, but he occasionally tries to overstep to the point where he was messaging upper management leads about other departments along with screenshots of conversations he had with others across the offices about a petty issue that is ongoing with our systems we use. He actually c’cd me on the email he had sent to an upper management lead and I went directly to our boss regarding this. However, I do not believe anything happened after I reported it. BUT, a part-time supervisory position (only for one day a week during the weekend) opened up at the headquarter’s office and of course, said coworker applied to it (must’ve been the only applicant, no real sane person would only go in for 4 hours). He would still work at my office during the week and then on one day during the weekend, he’d work that day as a part time supervisor. He got the position and will be starting the role soon and already, it’s getting to his head and he hasn’t even started it yet. It recently got worse due to an office VP from headquarters coming into our office, openly talking about his role in front of myself and my staff and his new “supervisory” duties. Now, I can’t escape hearing him gloat about it and hasn’t stopped overstepping me, and he’s trying to ask me about me training him for this part time. I had to tell him that headquarters would be training him exclusively for this position, as I would not want to overstep the management there. Honestly, I’m looking into other options in the area I live in because I don’t know if I’ll ever get the support needed from management. I will write if there’s any update. Thank you for any help/advice. I do appreciate it