Good for you. He should have asked for his birthcertificate and had everything in place way before that. I know how difficult child's work, but hopefully this will teach him next time that he has to step up and take care of these things himself, because mommy isn't going to be there to do it for him. It takes some getting used too, but believe me, they do figure it out. Mine has. What if you didn't have a copy of his birth certificate at home? He just assumed you did, but sometimes it takes a trip to town hall.
Of course this will be your fault and your reply should be, sorry you feel that way, but you had plenty of time to get this in place.