You did everything you could. Unfortunately while you gave him several chances at being safe around the rest of the people, the gun however is not so forgiving. If you want to know my honest opinion, and if I was instructing, I would've given the expectations of the class, which I am certain you did, and stood by them. It isn't personal, it's VERY PERSONAL. If it wasn't for the responsible weapons owners, we'd not have the privilege and right to have them.