My .02 cents. I have been in both situations. I am now 51. I see this as being how the kid was raised meaning his parents are to blame for not having instilled the common sense and courtesy into this kid.
First off. Have a heart to heart personal chat with the lad. Explaing how things were BEFORE he arrived. The agreement between the owner and the 2 other hunters. Now he has arrived. New mix. Same deal, another hunter.
Second option. If after first option he does come around and at least respect that you guys were there first and established areas due to first come first served, you take it up with the landowner and explain what ya'll have done to work with this kid and that he feels and acts like he owns the place and see what the owner says and does. Especially that being you and the 2nd hunter more or less work for the landowner.