Retired from ridden work could meaning things; he could have been sold on as a companion, taken on by a rescue, etc, etc. I think it's important to know what you'd do with the info, are you hoping to have him back? Or do you just want to know what's happened to him? How would you feel if he'd since been PTS? Or if your search for him proved fruitless,?
There's no harm in contacting the person you sold him on to, just to enquire