If your son keeps the firearm in his own room, locked in a gun safe which your husband has no access to, your husband wouldn't seem to be in violation of any federal law. If your son leaves the handgun in the coffee table right next to where your husband watches football games, that's a bigger problem. In between those situations are a million other possible fact patterns.
No federal law that I know of prohibits your husband from being in the same home as the firearm - POSSESSION is the problem. "Normal" possession is when you have the thing on you. However, under the law, possession can also be as simple as knowledge of the item and the ability or intent to control it.
California has its own restrictions on firearms, but I don't know of any state that would prohibit "being in the same house" or "being within 100 feet" of a firearm.
The dealer you son purchases his handgun from is likely familiar with the relevant laws. Doesn't make him an attorney, but he may be able to provide some guidance short of getting an opinion from an attorney.
My suspicion is that security guard classes do not teach proper handling of firearms and the laws regarding lawful use of firearms. I suspect that is required for the carry permit, however.
Moreover, he CANNOT have it if you tell him it isn't a good idea right now. The firearms "climate" is about to get very, very cold. Why have your husband risk a problem?
Given the year you cited, your husband is likely not on parole. If he somehow IS on parole, this is a big NOPE. Do NOT bring a firearm into your home.