Is he any particualr breed? If so it's best to contact either the british verision of that breed society or the breed society where ever it is and they can then transfer it to an English one with your name.
Bit from their website
The BHS as a PIO
The BHS is one of over 70 Passport Issuing Organisations. We aim to provide this service to owners of equines of unknown origin, that are not eligible to be registered with a recognised breed society . If you would like to apply through the BHS please send a SAE to The Passport Department, The British Horse Society, Stoneligh Deer Park, Kenilworth, Warwickshire, CV8 2XZ. The BHS ensures every effort will be made to return your Equine Passport as soon as possible, application turnaround time is approximately two weeks
Has he not got a brand or a number brand or anything to give you more of a clue?
Thank you very much, I will contact the BHS today. He has no brands or numbers on him at all, just a white mark around his neck where he was tethered and it has cut into him.
my pony had a german passposrt and after ringing the bhs, the passport people i rung the British FEI. They told me i needed to ring the German FEI and they then gave me their number. i rung them and i sent them my passport and i had it back with in 3 weeks! i think the FEI would be your best bet.