Get him registered with SHBGB as they will give you a passport that includes his pedigree if you supply his reg papers. If you are willing to risk sending them back to Holland then contact KWPN or NRPS as appropriate so that then you have a passport provided by the original registering studbook.
AS your horse is Dutch bred he will have full breeding details known and if you use an ID-only PIO such as HPA, Pet-ID, Veteran Horse Society, BHS etc (instead of a studbook PIO such as SHBGB, BWBS or the Dutch ones mentioned above) you will not have this vital information on your passport which is not a good idea :-(.
BTW, vets do not issue passports. They act as agents for commercial ID-only PIOs (eg HPA, Pet-ID, VHS etc) and get a percentage of the fees for the ones they send in. Some unscrupulous vets and ID-only PIOs therefore try to get people to believe that only they can issue DEFRA approved passports when in fact they are far better, more valuable -- and usually far more accurate -- if you can get them from a studbook PIO.