Who pays when someone drunk borrows your car in an accident?

My son was hit by a drunk driver. The driver did a hit and run. My son started having headaches, throwing up, feeling dizzy, sleeping a lot. The doctor concluded he had a concussion. My son also had a huge scar on his forehead and a lump. The lumo went away after about a month. The scar is still there. Anyhow, he went to the neurologist for treatment. About 4 months later, the neuro released him. He drove to a friend's house, on his way, he passed out and totaled out his car. He went back to the neuro and the neuro said his concussion may not have been completely healed or the last accident has triggered something going off in his brain. She said he could be having seizures. He was supposed to try to work full time, but ever since that accident he hasnt been right. My question, if he has debilitating possibly life altering injuries, because we dont know when/ if he can drive again. What can he do?  The driver got charged with DUI. This was a hit and run. He was driving someone else car with permission. The driver has to take responsibility, but my question if his injuries become life altering and his insurance company cant cover the cost. Do they go after the guys insurance, too? I ask because my son is not the same, and he is only 21.