Why is your business so dependent on this one person - what if he decides to quit of his own accord, or becomes sick, or has an accident?

I think you should plan as if he were to suddenly become sick - e.g: get any passwords, or access that he has made generic, or transfer ownership or admin rights to you, so that you aren't locked out of your systems, any information that he has needs to be made available, whether that's writing it down, or getting him to record his screen using something like Loom - so that you have processes documented, and put on a shared drive?

In terms of his behaviour, you could either speak to him about it, maybe things have changed in his personal life and its affecting him at work, or decide that the best course of action is to let him go.

Hope that helps, and I'd be happy to chat further on a call if you wish!