I would try contacting the woman you sold him to, to see if she still has him. It looks like she was last active in 2010 on the board listed, but maybe if you post there someone will have some current contact info. Best of luck!
There is a specific column called "Missing Horses" on this forum, at the top of the forums page. It is for horses that people have lost track of, as well as lost, or stolen.... Perhaps you should post there, and might have better luck.