How much should I charge for the use of my well?

Meter the power and charge him the whole nut or have him give you points on his cattle profits plus the pro rated power.

I'd be skeptical about this. Your neighbor seems like one of those pushy types who takes advantage of people. He's stealing now. What happens when the land changes possession through sale or death? Suppose he doesn't pay? Is there a shutoff valve for his cattle's water on your property? Why can't he dig his own well? This sounds like there is potential for serious problems. Maybe it would be worth your time and money to talk to a lawyer. There is something called adverse possession. Ask about it. There's an old adage. Good fences make good neighbors.

I agree. I would be reluctant to get into a sharing agreement with anyone, and very reluctant to get into one with a neighbor that already tapped into my well without permission. I see a lot of aggrivation and problems. What gave the neighbor the idea he was entitled to the water to begin with? One well served two homes and the property was divided and one half sold off? If so, the well issue should have been spelled out in the contract. If it's just land, no structure, then IDK what would give the neighbor the idea that he could tap into the well.

Turn off the power and go on vacation for a week or two. I did this thirty years ago and came home to backhoes and machines digging.. oops.