Idaho is one of the "Gold Star" open carry states listed on this websites maps.
Open carry is legal. However, trespassing is not, and you as the Property owner have every right to refuse to allow anything you don;t want on your property.
You could bar green eyes if you wanted.
So.. the kid is right. He can carry. And you are right. He can't carry on YOUR land, without your permission.
Easy enough to find on this great site.
Welcome aboard, btw.