Seems like he has been given more respect than he deserves.
First of get them off the rd. Get to know the land and put the father and son on a spot where they will see deer and have a good chance for the boy to harverst/kill deer. Make sure it's not close to an access point that you will use. For yours and your GF's safety.
They have permission. They don't have the run of the land. Work with them and not against them, but be firm. Make sure it's OK with your GF's parents too. Take control now and you will have a honey hole in a few yrs.

Here's a scenario for you. I got permission on a piece of land this yr. The land owner allows 2 groups(small groups) in a yr. In the past it was a buddy of mine friom work and a local guy. My buddy has had problems with the local guy for yrs. Always asking to see written permission and telling him to get off the land and such. The local guy treated the land as his own even though he didn't own the property. Well this yr I got permission through his daughter b4 he did. When he asked for permision he was snubbed. Landowner didn't call him back. I met him one night and boy was he kissing BUTT. The landowner has never asked for lease fees b4....... now the local guy tells me its $100 a day to hunt....... for him.I talked to the landowner and told him the guy was nice to me...... he says the $100 a day still stands for him. Not for me. No leaase fee's nothing.
I communicate with the other guys on the land and I also help him and his family out. His daughter has never tried deer b4...... I started with sausage and then went to giving her a bag of meat with instructions on how I cook it. I gave her a strip of loin, top sirloin and a sirloin tip. When she said she liked it I told hewr she could have half of the next deer I shot...... shoulder cuts will take some time to get to you aas thats what I use for sausage. I'll throw the pork in and make them. I got a nice doe and gave her meat. I also told her that her shouldeer weighed 10lbs........ and she'd get 20lbs of sausage. She offered me money fopr the pork. I said you did me a favour now let me do one for you. PS I also straightened her garage.AND found a natural medicine for her father's joint pain. Plus I went out and cut up a bush chord of wood for his banker of 30yrs. I go out opf my way to help the landowner...... Its his land and I have been given the right to hunt his land........ it can be taken away just as fast.
Sorry for the long post[:-]