I am not a lawyer so I don't want to speak with confidence here. I quickly Googled it and here is
an article on the shooting that mentions the exemption
Though its ambiguous its possible the exemption applies and I imagine his lawyers will try to argue that. Let's say worst case scenario it doesn't apply and he's guilty of a Class A misdemeanor, I don't think that would undermine his self defense argument. Breaking the law only undermines self defense if the law you broke provoked the attack but I don't think its fair to argue that a kid who is almost old enough to legally open carry but isn't is doing something that inherently or reasonably provokes an attack.
So I don't see why they can't, at worst, convict him of the misdemeanor(maximum punishment being 9 months in jail and/or a $10,000) and acquit him of homicide. Again that's if the exemption doesn't apply which if it does he can get off Scot free and even if it doesn't that doesn't mean he gets the maximum punishment.