Updated Opinion.....spoilers

In that ending scene with Gary rubbing it in David's face on who the real dad was perhaps this is the meaning of the film.

It might be showing that David had his issues, but the child made there lives whole it heeled/helped him deal with his abusive past. Nick was gone, The mom was dead, Gary was out of there lives, He and the wife had reconciled remember this is 18 months down the road.

When the evil person in Gary whom did them both in including Nick by being with his love Sinead tried to bring him back into a mode he had gotten past [brooding damaged] and the wife is caught in her lies he gets angry for just a moment, but then looks at his child as if to say you will get better treatment than we did[ he and Nick]. Smiles and stays the person he became in those 18 months. So maybe in the end it is David whom is the one who does well/grow and is more of a survivor of abuse than we are giving credit to. He seemed to forgive all in one big swoop and seems to have moved on.