You or your boyfriend would need to check with the California Department of Motor Vehicles to see what would happen if he was pulled over while driving your car with his expired tourist visa.

Normally with a valid foreign license (not just the international driving permit) you can drive in the United States, including California, for up to a year. If it has been over that period of time then they may consider him to be "unlicensed" but you should check with the CA DMV to find out for certain how they categorize this and what penalties would be applied.

California recognizes a valid driver license that is issued by a foreign jurisdiction (country, state, territory) of which the license holder is a resident but does not recognize an International Driving Permit (IDP) as a valid driver license.

The IDP is only a translation of information contained on a person's foreign driver license and is not required to operate a motor vehicle in California. Citations issued to a person in California who has an IDP, but does not have a California driver license will be placed on the Department of Motor Vehicle database.