You're buying his time and experience for agreed upon hours. You don't have the ability to make him put your work ahead of his anymore than you have the ability to make him put your work ahead of his family, personal life, education or retirement savings.
In discussions like this I tend to fall on sympathizing with employers because until you've taken the risk, hired and managed people you don't fully appreciate just how hard it is or how difficult it is to keep people happy.
What you're describing is closer to buying his life - which would be an understandable arrangement from a business partner but not an everyday employee.