New to reddit, so please tell me if I am in the wrong place to ask.

This post is not for me, but for my partner who failed to register for the selective service before his 26th birthday. It has come back to bite him in the ass as he is not eligible to receive financial aid for school. He was not aware of it at the age of 18, or any year after that until the financial aid office at school contacted him to submit proof of registration. I've discovered that he can appeal this punishment if he can prove that his failure was "not knowing and willful". However we are at a loss as to how to prove he was not aware. Any leads or advice would be much appreciated!