He did not register because he was not a citizen at the time.
Later he decided he was a citizen, but was then too old to register. Case closed.
/s
That seems more likely as there is nothing in the current Selective Service law which implies that you lose your citizenship for failure to comply. Only that you won’t be eligible for Federal student aid of a Federal job.
In fact the law says all male NON-US CITIZENS living in the US on their 18th birthday must register. Including ILLEGAL ALIENS!!!
Yeah, SURE they are.