I see five options here:
Option one: It is clear that this guy is going to do nothing about the situation and continuing to stew over it is not productive. You had mentioned that your property value has dropped over 100K because of the dump next door. If that is truly the case, then consider pricing out what it would cost to clean the mess up. Figure four 40-yard containers at 750 each, a bobcat with grapple bucket and excavator with a thumb should be sufficient to tame that mess. A local company charges 1K per day for equipment rental which includes an operator. Throw in a few day laborers, perhaps a grand worth. So, my guess is 3000 for the dumpsters, 4000 for equpment rental and a grand for the laborers, 8000 to make this go away. You are now down 8 grand instead of 100.
Option two, the great American lawsuit. That is a nuclear option that will cause serious, irreversible problems with this guy.
Option Three, the fence.
Option 4, learn to live with it
Option 5, move