The State must prove that the gun belonged to your son or at least that he was aware that it was in the car and was capable of exercising control over it--what the law calls "constructive possession." You didn't say in your question where the gun was located in the car or whose car it was, but the more hidden the gun was, the more difficult it will be for the State to prove that your son knew it was there. As for your son finding out later who the gun belonged to and informing the police, there are two issues: (1) Any statement by your son that the gun belonged to someone else in the car will sound self-serving and is not likely to be believed by the police unless there is something to corroborate it; and (2) It is not possible to predict whether your son might face possible retribution by the real gun owner if he implicates him.
Answered on Dec 03rd, 2019 at 11:23 AM