Does the questioner fall under the threat of forsaking a Muslim, knowing that the other party is the one who swore not to speak to him and slanders his reputation? Is it permissible for him to ask his friend, the employer, to fire this man for denying a religion to him? Is it permissible for him to supplicate against him in prayer?

It would have been more complete for you to be patient and forgive the young man who assaulted you, seeking reward from Allah, and it is not permissible for you to shun him for more than three days. However, if you see a legitimate benefit in shunning him, or fear harm in speaking with him, then you may do so. His denial of your right against him is considered an injustice, and you are permitted to reclaim your right from him through a lawful means, but seeking to have him expelled from his job is not permissible.