This is a tough one but I think it would be beneficial to let this one go. The pain you're experiencing is your own. A licensed therapist/psychiatrist (I live with one) would advise against scolding or correcting the child. It's too confusing. When he's older he will know the difference between Dad and Step-Dad, and even if he continues to call this other guy Dad maybe that's good. Maybe they have a healthy relationship and that can't be a bad thing, right? I'm in the same situation with my three year old daughter and I can attest to it being a difficult pill to swallow. Good luck!