Seems like the two responses you have already received are the extreme opposites. One accuses you of lying, the other has personal experience with being somewhat traumatized.
I would think that, if you have a good relationship with your son, and he already has confusion about his sisters, this is the perfect time--both because of his age and the changing situation in your family--to discuss ALL the ways people can be family. He is old enough to know/learn some very basic "where do babies come from" stuff, so without going too deeply into sexuality, you can explain that his body got 'made' by you and another father. This person gave him his body heritage, so there may be things about his body and personality as he grows up that don't seem to match others in his family, and those are probably things that let him know something about his bio-dad. Tell him whatever wonderful things you can remember about his bio-dad, whatever made you want to be sexual with him and have a baby with him in the first place. Don't get into much about why you aren't with him anymore.
Then you can also describe the relationship he has with your ex-husband as a "life dad", someone who thinks of him as a son, and always has, so he can love him and keep calling him "Dad" since that is who has been there for him and helped him grow up.
Talking about your new husband can be more about how a new family comes together and sees itself as one strong thing. Your son is really old enough to be part of the decision about whether he is adopted or not, so you may want to get together as a family with a family therapist who works with blended families to help your son see why you and your fiance want to do this and help him make the choice without a lot of emotional pressure one way or the other. Maybe include your ex in the discussion at some point.
Ultimately, the goal is to make your son know that he is so lucky because he has THREE fathers and not just one. And that he is loved and supported.