Does his SIN start with 9 or with something else? If it starts with 9, it s a temporary SIN issued for a temporary resident. If it starts with something else, he must be a PR. Does he have any documents since back then?
If he wants to figure out what his position is, he must at some point talk to immigration about it. Maybe he should talk to a lawyer to find out if there is a discreet way he can ask about his status without risking anything. If for some reason, he never got PR and has lived in Canada the whole time illegally, he could get deported if immigration finds out about him. Is is married or living in a common law relationship with a Canadian? If that were true, in case he is not PR, he could be sponsored. If he is not, he might be able to apply on humane and compassionate grounds based on that his children are in Canada.