Well assuming the facts are as presented, then this guy is screwed.
First the search of the vehicle was incident to arrest and therefore legal. So he's going to claim the cops planted evidence? Doubtful, because they won't want to lose their job, go to jail, and get sued.
Second, you say his claim is that he carried the gun for protection from the bar to the bank. Then he claims he would unload the gun, take the ammo inside to his home, but leave the gun in the car on nights he wasn't making a deposit. No prosecutor, judge, or jury is likely to buy that. If he took the ammo inside why didn't he take the gun inside?
Also, he remembers that he unloaded the gun, but he forgot the gun was in the glovebox? Again, going to have a hard to time to get anyone to believe that story.
As others have said, drinking and driving, and drinking and guns don't mix.