Will my son lose his survivor benefits if my fiance adopts him?

Can you ask as an anonymous fan question: My son’s father passed away when he was six months old, and his bio dad was never on the birth certificate. My son collects social security survivors benefits from his bio father, and my question is if my fiancé after we get married adopts my son, will he lose those benefits? We don’t make a lot of money, and those benefits help with raising my son, and it would impact our household income greatly of my son were to lose those benefits. What’s the process of adoption when the father is deceased and never on the birth certificate?