There is a possibility he does have amnesia. Something caused him to drive off the road. Something caused him to stay in Buttonwillow for 10+ hours. Something was going on as to why he wanted to come home and talk to his parents. And if he was awake for 4 days, his mind had to be fragile. Perhaps the accident was the last straw? Or he hit his head hard enough to not know who he is.
He could have gotten out of the vehicle, walked to the highway, hitched a ride and could be anywhere. Or he went to Valencia and is sleeping in secluded areas, undetected. There are many stories of people who drop off the face of the Earth and survive. A lot of people will help him, give him rides, give him something to eat, offer him a place to stay. He might not know to go to a hospital. It just depends on what is wrong with him. FWIW, IMO.