wrong, It is not the law in TX or VA for my FLA registered vehicle to have a front tag. FWIW: my son drove his car for four years while in school in TX with his FLA registered vehicle and never was there any question of his legality in doing it. for 6 months upon purchase of my first tesla it had only a rear NJ tag on it because the car was in FLA the entire time and FLA does not require a front tag on a vehicle.
It would be in the great regulatory state of California. Your son would have had to register the car in California, get CA plates, and put both on. There's even an anonymous tattle tale form page on the DMV website for people to tattle on cars with out of state plates but are in CA beyond the prescribed amount of time. The state highway patrol is pretty strict about registration and plates as well since I believe the great bulk (if not all) their budget comes from registration fees (not tickets).