You should meet the guy. Not via phone or email, but actually meet the guy.
If this place is a worthwhile spot to hunt and this guy has had exclusive access for so long, he hasn't been taking care of his landowner. From the sound of the story the uncle was very flippant about giving permission to you guys and allowing you to run the other guy off. There's got to be more to the story with these two. Don't get yourselves wrapped up in the middle of a fight between the landowner and the other party. Even though the uncle doesn't want to get involved with "hunting stuff" he still needs to offer a final word to the guy if he's no longer allowed in there. This isn't your domain.
Also, as stated above, it's a lot easier to get along with a fellow hunter in the neighborhood if possible. This guy is going to feel slighted. He's also going to be a little defensive because you're holding the cards. If it turns out that you can't come to an amicable solution so that both parties can hunt the ground together don't worsen the situation if you can help it. Sour grapes in a situation like this could lead to all kinds of discontent. Interference with hunts, trespassing, retaliation, etc. are all possibilities. Ideally both sides are above this kind of behavior but it's something to keep in mind.