Adopting a Stepson

Hi Sara, my name is Jennifer, I might be in a hard situation and was wondering if you can give me some advice. I have a 2 and a half year old son whose biological father left me when I was 2 months pregnant. He has never called to find out how his son is, has never seen him. I have been with my fiance for over 2 years now and we have an 8 month old together. My older son has been calling my fiance his daddy, and thinks he is his dad; my fiance and I would like him to adopt him. My son doesn't know anything about his biological father and I'm scared to tell him about him. My dad ran into my sons biological uncle recently and was told that my sons dad, after almost 3 years apart, wants to see him. I am very upset about the whole thing and am terrified for some stranger to just walk into my sons life like that. Would you know anything about what we need to do in order for my fiance to adopt my son?Please and thank you,Jen