Help! Simple question about hiring an o/o

I searched so please dont flame me if this has already been answered.

A guy wants to drive for me. He has his own equipment (a rather nice hotshot setup). However, he does not want to pay for his insurance with my provider. He wants to use his own insurance....which appears to be appropriate. He has 1,000,000 in lilability and 100k cargo.....and he is very adamant that it is NOT non-trucking liability.

Is it possible to bring his insurance onto my authority? Ive asked around and some people say that he HAS to be on my liability at the very least. But it just doesnt make sense to me that if he has the appropriate coverage then why would he have to be on my liability.

How does this work?