Forgiveness is just another word. Whether you forgive him or not, does not change things in him. It has already been done. Cannot be undone.
It is for your personal benefit that you should consider. Harboring resentment and hatred for whatever reason is like retaining the poison inside our mind. With this poison, the person cannot function healthily and happily. So long as this poison is not rid of, this person suffers from self torture and torment. It is a very destabilising experience.
It is only for the sake of personal well-being that the person "let go" of the hateful and unforgiving thoughts. By doing so, the poison is purged, freeing the person to live a new life of freedom. For this reason alone, you may forgive him.