Yes, he can be fined and go to jail, but you can be fined for work done without a permit. If someone is injured by some dangerous condition of the property, you are also potentially liable as well. Work over a few hundred dollars requires a building permit.
Building permits can be obtained by the homeowner or a licensed contractor. The person or persons performing the work must obtain the permit. It is a violation of state law if a contractor commences work without first obtaining a permit under their state contractor's license.
What Kind of Lawyer Do I Need to Sue a Contractor? If you are considering suing your California contractor, it is important to consult with a California contract lawyer.