Contacting PtHA is a good idea, however, you simply need to put "unknown" for the name of sire & dam and leave the rest blank. Your pinto certificate won't have a pedigree on it. You will have to check the "undocumented" box and pay the $125 registration fee plus your membership fee.
Put as much information as you know about his birthdate & gelding date. I'm not sure how they handle that, but I would bet they choose January 1st of the year you indicate or possibly the first day of the month/year you put. It really doesn't matter as long as you submit your photos and fees.
I hope you enjoy your new pinto!
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