We are a small trucking company which has a driver wanting to lease on with us. He has already purchased the liability insurance for his truck and wants to continue to use that. He says that the company he was with before allowed him to run under his own insurance but I don't believe that is legal. Does anyone have any knowledge on this subject? Thank you.

I don't think it would be legal or even a good idea because now when somebody sues they sue everything and everyone involved. Like if someone eats at a mall and they get sick they sue the restaurant they ate at, the food company they got sick on and the mall.
If it is legal and you hire him you should get an LLC license if you don't already have on.
May be a good idea even if you do not hire him.
Here is a site I think should answer your ?'s
http://www.wsib.on.ca/wsib/wsibsite.nsf/LookupFiles/DownloadableFileTransportationIndependentOperator/$File/TransIO.pdf

Don't know if this would be of any interest to you.

loadcator.com - Home

I think that he can have his own liability insurance for the rig itself, but for the trailer and contents, that would be your responsibility. I would suggest contacting your insurance company.