My understanding is that all adoptions run the same way, regardless of whether the adopting parent knows the child. You would go through the same channels as if you were both seeking to adopt a newborn who was unknown to you.
Start with DCFS (Division of Child & Family Services). While procedures can't be bypassed, sometimes the legal fees can be reduced significantly if you work with your state agency. In Illinois, all prospective adopting parents need to be certified to be foster parents, and there are classes and investigations surrounding that.
Once your fiance adopts your son, he will be issued a new birth certificate with your fiance listed as his father. And your son's name can be changed then; it's usually done that way, as a '2-for-1' deal.
But you might need a parental release from his bio father, depending how long it's been since he abandoned you/your son. Generally it's 7 years, after which you don't need permission. But check with the agency and see what they say.
Good luck, and congratulations.
A.