It is possible that he had a short certificate from his original birth registration which would name him as Harold Arthur Forrest KOSELJ and he would simply tell whoever he presented it to that he was only known by the first three names. He would be quite entitled to do that as a personal preference as long as he wasn't doing it to commit fraud. This short certificate would not have parents' names on it though.
In another scenario, if they attempted to have a search done for an original birth registration but were unable to locate it, you would expect to see a late registration in the indexes in order for a birth certificate to be issued. There doesn't seem to be one.
Are you sure it was a birth certificate and not a certificate of baptism?
Debra