Insrance required for offroad vehicle?

My son was pulled over by our local Sheriff's Dept on a 2 wheel dirtbike. He had just bought it that day from some guy(???????). He went to the river bed riding and was going back to his Grandmothers house. He rode through an orange grove then crossed over a street ( while still riding the motorcycle). He was on the public roadway for about 3 house lengths(approx. 1/32 of a mile?). My son was 17 at the time and not yet obtained his license. The officer cited him for unlicensed driver, Failure to provide a peace officer with proof of insurance, and No evidence of current registration. Can someone please tell me if these charges are arguable? Can an officer actually charge an offroad vehicle these counts? I understand the unlicensed driver charge since the laws pertaining to an off-highway vehicle change to the Vehicle code when operated on a public roadway. But, do they also pertain to insurance?