Loaded firearm in California, not concealed?

There are not enough facts here to tell whether this is a valid charge or not, but if it is valid it is not for any reason stated by the officer. Whether it is lawful to have a loaded weapon has nothing to do with whether the firearm can be concealed. It is lawful to have a loaded weapon on your own property, but also at your place of business, at any temporary camp site, and in any place outside city limits where shooting is permitted. If he was within a state or national park or forest other rules might apply. If a criminal charge is actually filed in this case (and if the weapon was lawfully possessed a prosecutor may decline to file a charge), he should unquestionably secure counsel. If he cannot afford an attorney, one will be appointed for him.