My father (90) is threatening to kill his grandson.

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.

I do hope grandfather hasn't any access to firearms??

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.