I bet if you start deducting storage fees from his deposit he will come get the car. If what you say is accurate, then just holding the car for four months with no action on his part is reason enough for me to start eating away at the deposit. I know he hasn't paid for it in full yet, but 4 months is a long time to keep it off the market AND give him his deposit back.
I also have to tell you that I have a $100 deposit from a man from 2 YEARS ago that never came to pick up a motor he was to purchase. I sold the motor a year later. You never know what the hell folks are thinking.