I think if you talk to him first and yall both look over the lines and work together it will all work out fine. People only get fired up "most of the time" when others cross lines without permission. If it is true about things getting stolen from his lease then its understandable that he may not trust anyone the land owner allows to hunt it. So good first impression is always the best.
Just b/c someone seems like a stand-up guy on an internet forum has no bearing on how he is in person, when he's trying to defend "his hunting grounds". I'd take the landowner's experience as truth and be careful about how you approach this person.
If you guys get along on the forum just message him or try to meet in person and have a calm regular conversation. If he flips out for no reason then you know he's just a jerk. I've been in a similar situation except these neighbors cross the road to get to the property I hunt and they denied what most people know they do. A few years back I found a busted up arrow that wasn't mine.
Yeah thats a tough situation. Maybe you could some how get him to figure out you are going to be neighbors on the forum by sharing your location. That way it's on his terms and he will superior for figuring it out. I don't know maybe you want anybody to know where it is though. We had a similar situation happen to us. We were hunting properties that bordered and owned by different people. Land owner A had agriculture land with some woods. Land owner B owned a pasture and a small slice of woods that bordered land owner A's woods. Land owner B repaired the fence but cut the corner of land owner A's property because he didn't want to fence through the winding creek. My dad, not trying to cause any trouble, brought it up the land owner A who was unaware and of the new fence. A court battle ensued and we lost permission on land owner B's property.
I am all about upfront and clearing the air. I would approach it like this. I would reach out to him stating that you recently got permission to hunt the other two tracts and you were informed he had leased the center tract. Just being neighborly you wanted to introduce yourself and discuss options. Options such as how should a situation be handled if you shoot a deer and it runs to his lease. That discussion can benefit both parties. Unfortunately perception is a cloudy subject. You see the lines as being a tree line or field and he sees the line as a creek. Discussing the boundaries is always a good thing to as other have pointed out so that you both are on the same page. Its always good to be neighborly until a last resort calls for you not to be. Good luck. :-)
And the sooner you do something the better. Don't put it off. You don't want any controversy, if there is any, to continue into the season. Could make for a miserable year.
THIS is your answer^^^ Also, when you put your video cameras up, do it in unusual ways that someone wouldn't suspect they are there. Video EVERYTHING--especially any encounters you may have with this individual. I wouldn't hesitate to iron all this out BEFORE the season. Who wants to deal with this while trying to hunt. You should also make the acquaintance of your local WCO and have their number in you cell phone. If it comes down to a deer on the ground, you may need backup. Everything is fine till that big-antlered, monster buck-of-a-lifetime gets added to the mix. Then the "crazy" comes out.