Does it help any to know that having things stolen is a fairly common delusion in dementia? Sometimes a specific person is accused and sometimes it is left vague. "They keep getting in at night and taking my things! Today a box of toothpicks is missing!" The good news is that this usually goes away on its own -- it doesn't last the entire course of the disease.
Tell him it's OK, that grandson froze to death in the woods last night. Then have grandson stay away a week or so until grandpa says he misses him. Remove all firearms from the house. If you cannot do this, ask the Wisconsin State Patrol to assist with this.
Your grandfather is suffering from paranoia. To be on the safe side, I would keep the grandson away from him until the stage passes. Also, get your grandfather to a doctor and a geriatric psychiatrist to see if you can get some relief from the paranoia. Paranoid people can be dangerous, so take what he says seriously.
Remove all firearms from the house. If you cannot do this, ask the Wisconsin State Patrol to assist with this.