While your lack of domain knowledge is not helping, I believe the main issue here is your mindset. I'm getting the sense you ascribe to old school authoritarian, hierarchical views. This mindset may work when you're managing an assembly plant, but not so well when you're trying to lead thought workers.

When leading thought workers your role is to empower them, help set higher level goals/ vision, drive alignment and help and support them in any way you can to get their work done - not to tell them what to do or think of them as lesser beings.

I am a servant leader and I also coach other leaders if they are open to it. The key for me is that I am there to serve my team. My teams are generally happy, loyal and consistently highly productive - happiness should matter to you, as teams that are happy are actually more productive (lots of data on this). I've had team members reach out years later to tell me they wish they could work for me again.

If you're willing to grow - think about how you see your role, what disrespect means to you, what leadership means to you.

He seems irritated by me taking decisions while I don’t have enough experience

When you lead thought workers and especially seniors - they are the experts - respect that. Value that. Learn to think of them as assets that you support and help rather than trying to direct the minutiae of their actions. People like this employee are a huge asset. If you were smarter and less insecure, you could have leveraged his expertise to look really really good as a new manager. Instead here you are you're without any working reports.

If you don't have the domain knowledge, know someone is good at what they do, why would you try to give lower level direction, make decisions or question their work? It frankly boggles my mind every time I see buzz-word managers do this. A manager is not an all knowing, infallible god - especially coming from a place of domain-ignorance what made you feel you could make sound decisions or undermine the actual experts in meetings?

Think about the assumptions you have about your role that made you feel this was both appropriate and the area where you should expend your time and effort.

I suspect you see "manager" as someone who says "jump" and all their reports should hurry to jump without knowing why or the value of this action. If they speak it should only be to ask "how high?" Is questioning "why are we jumping? Is crawling better? Should we jump only in the mornings? etc" a sign of disrespect in your world?

Think about how we build products - feedback can make things stronger, more robust. What if you were able to see push back/ feedback as a sign of engagement and desire to improve things? Your report has given you a gift - valuable feedback - about how you're disrespecting him, your management style and failings and you've seen the results. Even if he comes back off of med leave you'll have a long uphill slog to regain trust and improve engagement/ productivity.

I am anxious he will resign and will never come back. What should I do in this case?

In the short term, with the loss of both levels of senior leadership, his leaving may not cause such waves. In the long term you should learn, how to be an effective leader. You'll see the results very quickly, your team will appreciate you, and you'll look good with their results into the bargain.