Fresh,
Here is my take on this, so I would also back this up with legal advice too as mentioned or consult with a DOT authority specialist. I have been told, which is open to interpretation that if I was contracted, I need a DOT # but if I was open to "for hire" hauling I needed an authority, two different things and my source is the USPS contracting services group.
I think you will need at least a dot # on the side of the truck plus the standard insurance coverage, $500K if I am not mistaken. Seeing you are new to this, the insurance may be more than you think so I would get at least a few quotes.
The DOT# is the first problem, the record keeping the responsibility for a number of requirements and the final liability falls on his shoulders exclusively regardless how much you use the truck. You have to be careful to ensure he understands this and being an employee also opens up a couple of other issues that a good lawyer can discuss with you and him directly and bluntly.
With that said, I would be careful on how you plan on doing this, seeing you are reaching into subleasing world with the equipment and you have to have contracts setup to CYA with liabilities and insurance. The LLC structure may not be the right thing to do and may not offer you a single bit of liability protection under the situation in your state.
Remember that the insurance company will have the final say so on this and it is not your responsibility to insure the truck, but the owner's.