How do I tell my boss the reason why I am burnt out?

Background: The other day my boss pulled me into a 1:1 and he told me he thought I was getting burnt out and that he feared I was going quit. Since that conversation he has been reducing my work load.

The work load isn’t the problem. The problem is that my coworker and I used used to run a tight ship. We didn’t miss a beat. Recently my boss hired on two complete morons and for the past several weeks I have spent most of my time fixing simply mistakes that they shouldn’t be making. I have spoken to my boss about this but nothing happens.

I really like the company that I work for and I don’t want to leave. But I don’t know how much longer I can hang in this situation.

So my question is: How do I tell my boss the reason I am getting burnt out and that he needs to do something about these guys he hired or he will lose me?