Neighbors new wall and water problem

I live on the side of a Sm. Mountain. My neighbor wants to live on a Flat piece of land. He has now flattened the land directly next to my house,and has built up the land just in front of it, with a 20 ft high retaining wall(of Rockand dirt) My problem is,now he is working on the side that we share. His side retaining wall is partially on top of the stone wall that splits the prop. As I stated on the side of a mnt. So we normally have a large amount of water run off. Now I will be having all the water run off from his land also. I don't want to be flooded any more than I am already. Isn't there some kind of space that should be between his BIG DIG and my property line? Is he responsible for supplying some kind of drainage on his side? As he is changing the natural slope of the land. Please help