Two thoughts (that you've probably already thought of):
1) Your post says "new drain tile". Is your neighbor expanding the drained (and now tillable) area? If so, he could be running afoul of some wetland protection rules, and the DNR would probably like to hear about it.
2) Rather than a battle with you, would the neighbor be agreeable to replacing and extending the new drain tile with a solid pipe that would go to your exit? You'd obviously lose that direct inflow, but it doesn't sound like it's much of an asset at the moment. He gets to drain his fields and continue to make money, and you get your pond back. Win/win.
