Of course he's his father!
I have been divorced after having a child and I've known many people who have gone through custody battles.
I know things change over time and from state to state but I felt like it was a stretch to say that he has no rights JUST because his sperm didn't fertilize her egg.
1. They both thought the baby was his from conception until the first SEVERAL years of his life had gone by on top of the fact that he lived as his father all that time.
2. Because he was there since conception and undoubtedly present at the delivery he would have been named as the father on the birth certificate.
These things would have had a lot more sway in real life than they do in the movie, not to mention the fact that Sam has only ever known him as his father.
When I got divorced and asked for custody they might have asked me to confirm that my ex-husband was indeed the biological father of my son, but I don't even remember that. What I do remember being asked repeatedly and seriously was whether or not he was named as the father on the birth certificate. They told me that was the most important factor regarding his rights.
Was anyone else thinking this during the movie?