
This case sounds very serious. I am sorry to hear about your son's condition. What I am about to say may sound cold but I assure you it is your best viable option.
Trucking Companies have large insurance policies. You could make a claim against your Mom's company if you can show that the truck driver (your mom) was negligent in having your son in the truck which resulted in him falling out and getting injured.
There are several great trucking attorneys in the country and I would be happy to assist you. Research which attorney you ultimately decide to hire ensuring they practice trucking law and traumatic brain injury law.
For instance, my web site is http://www.titololawoffice.com. I also publish several blogs. The Brain and Spine Injury Law Blog, Truck Accident Blog, Las Vegas Injury Attorney Blog, and Tim Titolo. You can visit any of these sites and get a good idea of the type of cases I handle regularly. You can also get to know me by seeing and hearing me.