While I dont agree with it at all sadly he is legally well within his rights. My neighbour does the same thing to us. We have palm trees on our side that regularly drop their fronds. He always puts the dead ones over the fence back onto our side. Really freaking annoying and not something I choose to do but ar for saying something I dont think I would bother. You will most likely get the law quoted to you! Just keep an eye on his overhanging trees and branches and make sure you repay the favour!