I was really curious how the APHA handles these types of situations, so I asked them (hope you don't mind) Here is there response:
In order to assist you in locating the horse you mention, the following information will be needed:
1. Registered name and number of the sire
2. Registered name and number of the dam
3. Last recorded owner
APHA currently has over 890,000 horses registered and we must have this information in order to significantly narrow the search.
Edited to add: I am sure if you could find out his registered name that they would be able to help you find the rest of the information - I know that's the hard part, though :(