Neighbor's new extension possibly being "overbuilt" - what can i do?

Neighbor is extending the house into the back yard with a full width, 3-story extension. FAR and rear yard depth allows him (sadly robbing me of light and air) to go back about 14-15 feet from the original rear wall, but seems like he’s actually building 18-20 feet back. Do I have any ability to check what he got approved for vs. what he’s building, and if he really is overbuilding, how can i cry foul??