It now seems to make much more sense that birth certificate related reason for not registering for selective service would be more likely tied to being an foreign alien at the age of 18 or 19 in the year 1980.
So, if the draft form is in fact a true forgery, then, the reasons for him not registering could only be one of the three:
1. He refused to register under a form of personal protest
2. He was not a citizen of the US at the time
3. He simply forgot to register.