Both you and your boyfriend will be in trouble. Arizona law makes it a Class Two Misdemeanor to display a license plate or registration card that is suspended or revoked. It is also a misdemeanor to fail to surrender a suspended license plate to MVD. If the car is still yours, you could be cited for those violations, which carry fines and possible jail.
If the sale has already been completed, your boyfriend was supposed to transfer title, which includes showing proof the car is insured, within ten days of the sale. If he gets stopped while operating the car, he'll be cited for displaying a suspended plate, not having current registration and operating a vehicle without insurance. The last two charges are civil violations, but they carry very large fines (around $500.00).