Huh? Where did you see that? His father was not entitled to a SSN. Only citizens and LPRs get those.
It is upthread on the document where it discusses his job in Hawaii at Dole. He was given permission to work 35 or so hours a week, so he legally had a SS no.
My first husband was here on a Swiss USA work exchange visa, for two years, working as a hotel chef, with legal SS number too, though he also was not a true immigrant.