H.W.
Hi T.,
I'll say this as the kid who was adopted by my mom's second husband: I think it's good for her to be asking you how you would deal with her son's bio-dad coming back into his life.
Here's the thing: it doesn't have to be a dealbreaker, okay? She's always going to have an ex, and the boy is always going to have a not-present dad. Is it awkward? Sure, but as that kid whose bio-dad was NEVER spoken of until I was nine and started asking questions (I am Caucasian and my adoptive dad is Filipino-- the differences were obvious)... it's great that she can appreciate some parts the boy's bio-dad to him.
She's not saying anything about her son's cute nose or bellybutton to upset you. She's saying this to reassure her son that there were likable things about his father. Once again, from my perspective growing up, all I heard were the negative things about my bio-dad that we had in common and that my mother couldn't stand. The fac that your girlfriend is giving her son *positive* messages about his father is a gift. I know you feel slighted, but it's worthwhile to take a moment and think about this:
What if his father was never brought up? Only discussed covertly, in order not to upset you? Is that really the 'walking on eggshells' relationship that you want with them, that they have to be guarded about something as elemental as his parentage?
He's still young and will eventually-- and sadly-- figure out that his father is not invested in him, doesn't care, and isn't going to be around for the long haul. You might consider going to a counselor or another mentor to talk about your feelings regarding this situation.
I'll say this: it's NOT inappropriate to want your child to have a positive perception of *who* they came from, who has made a part of them. Sure, the guy is irresponsible, left her high and dry-- whatever. The point is that he is FIVE now and his self-esteem is still going to be developing for a long, long time. He will come to that point of questioning why his father wasn't around, wasn't available, and it's going to hurt like hell. Don't wish that on him just yet. If you are invested and looking to become a stepdad, then make the commitment to your relationship with him that you will let him take the lead on this one and honor that process. It's going to be a HARD one if you are openly disapproving of his dad.
Instead, if you can find empathy during those times he might mention his father, you can help the child feel loved and that he belongs in the family his mother is making now. "Yeah, I know you would like to meet him. He'd be proud to know what a great kid you are." I grew up not knowing my own bio-dad until I was 14, when I initiated contact. I will say this-- stepparents can do a lot and have lots of good influence. I'm 43 now, and nearly 30 years later, my relationship with my bio-dad's family has been mostly facilitated by my stepmom, and she's the one I still have the primary relationship with. You can be a powerful and good influence if you don't take these mentions of your girlfriend's ex as slights toward yourself, but reassurance for her son. If you feel you can't let it go, as I stated before, get some support through a counselor who specializes in blending families or from friends who have dealt with similar experiences.
Good luck.