I am confused, is he part of your Prepping group or just some guy/owner/Land Barron would be the tittle after SHTF, maybe?
A contract can be done by an attorney in real estate law, I would get one if your going to play in someone else land, last thing you want is him getting ****ed at you for one reason or another and going all "you kids get off my land."
After a SHTF contracts will be meaningless and of no use, he can pretty much say NO to your group, and you would have no real recourse except well leave
I am not a fan of people I don't know or trust been the owners of a place I am going to bug out to. I rather have 1 archer to go to; than a 1000 that is not mine to start with. But that is just me.