Hmm . . . What colour is the passport cover?
The Dartmoor Pony Society part-bred register issue yellow passports. To qualify, the pony must have a minimum of 12.5% pedigree Dartmoor blood (ie. A grandparent).
They will issue passports for non-pedigree ponies of unknown breeding and not eligible for entry into any breed society stud book. These passports are pink.
Not sure if that helps. It doesn't get you any closer to finding out your pony's parentage, but it might explain why he is registered with the Dartmoor Pony Society (if his passport is either yellow or pink, that is).
With his looks, I'm surprised they didn't register him with the Gypsy Pony and Cob Society. It would be a lot less confusing!