You are misreading what you are posting. There's a difference between being a relative (i.e. having some sort of family connection), being a relative who can sponsor (USC parent, USC spouse, USC son/daughter, USC sibling over 21, LPR parent, LPR spouse) and being a qualified relative for a waiver, which for fraud waivers is limited solely to USC/LPR spouses and parents of the applicant/beneficiary.
No idea where you are getting that:
1. He's not a biological son
2. That'd matter in the first place, adopted children can sponsor their parents, as can children that were legitimated despite lack of a blood relation, as can stepchildren as long as the marriage was entered before the child's 18th birthday, divorce or death of the parent doesn't automatically terminate a stepchild-stepparent relationship either.
Moreover, the sponsor and the qualifying relative do not have to be the same person. So it is possible for a USC son/daughter to petition the mother in IR5 and for her LPR spouse to be the qualifying relative for a waiver. Doesn't even have to be a family sponsorship category.