Key Words= Public Land. Unless you specifically asked him to stay clear of the spots you tend to hunt (they are not "your spots") then you don't even have an argument. Should it be common sence to him? Yes, but it's obviously not. Talk to him man to man and don't worry about the family stuff.
If I were you I would not tell him you were poking around in his GPS. Thats like looking through a dudes wallet. It's private and you got no business doing it. He may turn Roy D. Mercer on you and give you a "good ole country ars woopin". Goin through another mans GPS without his permission...