
If you contact the National dog Tattoo register they will have a registered name for him, iof he was already registered when the tattoo was don, but at least it should give name of the breeder who had the pups tatttooed, or the owner if he was done later!
If it is the breeder, they may be willing to contact, or pass on a note from you, for informnation. Obviously they won't just give out the deatils without permission.
If he was KC registered, the KC will be able to tell you his parents names from the record they have, but not sure if the6y would give you a chance to buy a pedigree! Though obviously you wouldn't get his Registration docs, as the rescue kept them.
If it was breed rescue, they may let you have a copy of his pedigree to satisfy your curiosity, if they have it!