Actually, what you're saying is not exactly true about him being liable for a load. First off, is he running under his own authority or leased to a carrier? If he has his own authority he already knows that nobody will put a load on his truck without him having a minimun amount of cargo insurance. 100K is the norm for this insurance but 150K will completely cover him pulling any load no matter what happens. Also, he needs to have a MINIMUM of 1 million dollars liability insurance on his truck/trailer if he's operating on his own authority.
If he is leased to a company it's different. They will provide him with cargo insurance and a certain amount of liability. He needs to check and see how much it is. He still is responsible for comp/collision coverage on his rig.
Incorporating is to protect yourself from general liability(Accidents etc..), not cargo claims. It is also to protect your assets from double taxation in many cases. While incorporation is a good measure of protection, it is not bullet proof. What your Dad needs to do is consult a good tax attorney, hopefully one who is familiar with incorporation, and pay the fee to pick the guy's brain for an hour. Make sure your Dad goes armed with a list of questions to ask. They attorney will be able to look at his situation and give him the best recommendation.
With all this in mind, I highly recommend the Owner Operators Independant Driver's Association (O.O.I.D.A.) out of Missouri. They have many resources, including legal, for their members. Membership is $45 a year and well worth it for a serious O/O. Look them up on the web, I think it's www.ooida.com or something close. Give your Dad their number and have him talk to them first. They are one of those rare organizations that stands behind drivers.