Problem is, I don't have a lawn yet. My back yard is still dirt, so the damage to his lawn comes from the dirt running off my yard into his during massive rain storms. I specificially avoided building a fence and blocking off drainage there due to waiting for the bulder to first replace his lawn. I'm getting the fence up now, as fence post holes are already dug.

He from what I have seen has done nothing on his side to address this. A large pile of dirt is sitting on top of a small (foot squared) area and he hasn't even attempted to move it, instead letting it kill the grass.