As I'm sure you know, as a felon, your fiance is not allowed anywhere near a firearm. Therefore, if he is in a situation where it may be possible for him to take custody and control over a firearm he can be charged with its construction possession. He can even be charged with it if he didn't know the gun was there, if it's reasonable for him to have known the gun was there and possible for him to take it into his custody and control.
Whether he can be convicted of this can be another story. It's not good enough that the firearms where yours. If he was driving the car, had the keys to the car and access to the trunk where the guns were, it doesn't matter whose guns were there. It's possible for a conviction.
If he was charged Federally, to defend against this he'd need a Federal lawyer. He'd be entitled as part of fighting this case to challenge the legitimacy of the search of the vehicle with a view to getting the evidence suppressed. Obviously, if the guns were suppressed there would be no case, because without the contraband the possession charge would fall.
If he loses the hearing, then it's the state's burden to try to prove beyond a reasonable doubt that he knew the guns were in the trunk and was capable of taking physical custody and control over them.