Put up an ad on Craigslist. Even if the person who now owns him doesn't see the ad, it might be that the farrier who works on his feet might see him, maybe a vet's assistant, maybe the neighbour of the owner. Be prepared for a lot of nuts to answer, though.
Something like: "Looking for a horse I used to own--sold to someone in this area. Do you recognise him? Please contact me, as I would like to know how he is doing."
Don't specify that you would like to buy him back--you will really get the nuts and scammers coming out of the woodwork with that! If the new owner contacts you, that's when you suggest that you would like him back.