Forgiveness is for after the fact, not during the act. I seem to recall Jesus getting a little angry on those people who were selling stuff in the temple, overturning tables and in a bit of a rage.
This father acted perfectly. In my eyes, he was perfectly in bounds to beat the perp to death. He showed great mercy in allowing him to live and face justice.
He also showed his son how much he loves and cares for him. That should help the kids healing just a bit.
No, unless he repents he has taught his son to find refuge in Satan.