Hi! First of all, your business can never depend on nobody, even if this someone is yourself.
It looks like your employee knows that he is essencial for your company, and it makes him powerfull.
If I were you, I would try to understand what is happening with this employee. He is human, not a robot, and everybody has problems in some moments of lives. Maybe he is going trought some difficult moments that are changing his behavior. It could be a disease, a family problem, a financial problem, or it’s just a problem with the job. However, the first step is have a professional conversation with your employee to understand what is going on.
If you the problem is the job, try to understand which facts are making him unmotivated and what could you do to make him feel better. Sometimes a salary rise or new beneficits will solve the problem. Sometimes you should review the company environment, culture, relationships…
But one thing is certain, you gotta have a honest and open mind conversation with him. Show him that he’s important (not dependent), reconized and valued for your company.