On more than one occassion, I've actually turned over all the coins that I've found on a piece of property. I call that creating Karma.
When I am asked if I will split my finds, I tell them that I spent a lot of money on my detector so it should find lots of stuff. I also tell them that if I find a pot of gold, I will split it with them as long as I get the first one. In addition, I'll haul off the nails, washers, shotgun shell casings, rifle casings, pop cans and any other stuff I find. I keep a bucket in the back of my truck where I put all the junk and I make an effort to show it to them. Once they see the amount of "crap" that can come from an old site, they usually laugh and tell me that I can keep what I find. Worst case scenerio, if I do happen to find one gold coin, it is mine! If I find two, I'll honor the agreement and we'll both be tickled. In the meantime, I'm hauling off all the nails, washers, shotgun shell casings, rifle shell casings, pop cans and all the other "stuff" that I found. That other "stuff" can include some pretty nice coins that aren't gold! But since they weren't part of the "split" agreement, I don't feel compelled to show them everything after the hunt.
In your situation, I'd tell him that you appreciate his allowing you to cross his property to access the first site and that you would like to detect his property after you have completed your initial quest. When you are ready to hunt his property, be aware that getting the "impression" that he wants half is different from entering into an agreement to split the finds. Don't just get an impression. Flat out ask him what he expects. If the land has been in his family for generations, ask him if he knows of anything that they may have lost over the years. Offer to give him any jewelry you find that he thinks may have been his ancestors. Other than that, you'll keep the rest. If he insists on getting half of everything you find, know up-front that half of everything includes those nails, washers, shotgun shell casings, rifle casings, pop cans and yes, even the coins. You might counter offer to give him half of everything you find except coins. And just in case you happen to find an old penny, you would be willing to give him "face value" for it. In other words, don't embellish what it is you expect to find. Personally, if he insists on getting half of everything, I'd draw up an informal contract spelling out what it is he has asked. But again, make sure the contract indicates that if you find just one of any denomination, it is yours. Splitting one quarter down the middle can be a very trying task! When it comes right down to signing it, I doubt he is really that interested in making you split the finds. Especially after you've shown him that bucket of junk in your truck! If he were really interested in finding old coins in a field, he would have already bought his own detector and you wouldn't be hunting those sites. JMHO HH Randy