I don't know how it is in CA, but there is also an insurance issue at stake here. If he were to have an accident, the insurance company could come after ya'll, as he knowingly drove in his "condition." We made the mistake shortly after my husband was diagnosed, in telling our agent about husband's illness. He was perfectly capable of driving at the time, but the insurance company required either a test from the motor vehicle dept. or a letter from a doctor. The neuro at the ALS clinic would not commit so we went to his Internal Med guy and he wrote the letter stating he was fine to drive.
Towards the end, my husband had slight dementia and drove on two occasions, when he should not have, justified in his own mine that he was OK in driving. He had not driven in so long that I was caught off guard that he had found a way to turn the key in the ignition. These episodes were months apart and tragedy was avoided. I can only imagine what kind of law suit I'd be dealing with to this day, if he had not been so lucky.
happygardener, if he is not suffering from dementia, you may be able to appeal to him from the insurance aspect, anyway. I wish you luck.