Forgiveness is not for the one being forgiven, it's for the one doing the forgiving. Forgiving someone is basically saying "I'm done hating you". It doesn't mean that you don't want them to face justice. When you feel like it's right to let go of your negative feelings is up to you. I'm an atheist so I'm not going to try to back any of this up with any religious writings or preachings nor am I going to comment on whether this man should forgive his son's killer or not, all I'll say is forgive when you feel it's best to do so, not for them but for yourself. And if you never feel like forgiving them then I feel sorry for you, holding that much hatred and anger, whether justified or not, is a painful way to live. And when you allow yourself to start healing is up to you.