You may own the firearm, but that doesn’t mean you’re the only person who can physically POSSESS the firearm. He was in the car (I’d wager in the passenger seat) there was a gun in the glove box, and he’s not allowed to have a gun. Constructive possession says that he was illegally in possession of the gun, and it looks like they intend to charge him for it. US Marshals don’t fuck around, if they’re after someone, he’s a bad dude, and they’re going to hit him with whatever charges they can, especially if it involves an illegal firearm. It sounds like they intend to charge him for it.

The question at this point isn’t whether you own the gun, you surely do, but whether he was in possession of it at the time of arrest. You can consult with an attorney, and he certainly needs one.