Difficult Remote Former Peer

I (38F) am now the director of my team which includes a (62M) remote employee. I include our ages and genders because he's pretty old fashioned and I think sees it as an insult to be reporting to me, even though he didn't want the promotion himself. He is a high producing employee and we've gotten along fairly well in the past. He has been resistant to O3s with me, insisting that I am trying to manage him and reduce the flexibility of his work arrangement, as he's essentially been unmanaged in the past. This isn't my main question, as I've listened to all of the podcasts on O3 pushback and I am trying not to make any changes in the first few months except for requesting O3s

He is easily offended at the smallest issues, often copying my boss and sometimes her boss as well on his inflammatory email responses. My boss and the CEO have a generous open door policy, so they won't tell him to stop escalating to them, but fortunately they haven't really engaged in return. He is very blunt, but if I'm blunt back to him, he'll continue to escalate (even though I'm sure he's a high D in DiSC). I think he wants me to fail and he knows that the CEO sees his contributions as essential to the company, so I believe that my role power would be somewhat limited even though I haven't tried to use it yet. 

Any suggestions on how I could de-escalate this situation and get the relationship on a better track? I'm a first-time director and am putting my best effort into doing it well--and it is going well with the rest of the team. To that end, I'm really grateful for Manager Tools and this community!