I'm not really an expert. I've had a big problem with sunburn with our heat out here. I now have an EZ up shade to protect the ones that would get no shade, so I'm hesitant to prune mine too much.

Here is a video from UC about pruning avocados: http://ucavo.ucr.edu/General/PruningSmallTree2.html

UPDATE: I didn't do anything to the tree and it has since grown new branches. It looks like a new strong central leader is starting to take shape.