All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We have already clarified in Fataawa 281386, 284845, 287968 that injustice is forbidden even if the person treated unjustly is a non-Muslim, and that a person must repent from injustice and seek forgiveness from anyone he has wronged.
If the injustice was related to something besides property, like the moral rights related to honor and the like, and you sought his forgiveness and he refused, then the only thing that you can do is supplicate for him as much as possible concerning matters that are permissible to supplicate for a non-Muslim, like guidance.
An-Nawawi said: “As regards supplicating for him (i.e. the non-Muslim) for guidance and converting to Islam, this is permissible.”
You can also supplicate for him to have worldly favors, as this is permissible according to the view of some scholars since it is permissible to give condolences over tragedies that non-Muslims experience by praying for them to receive something in place of the thing lost or the like.
Hopefully Allaah will make him satisfied with you on the Day of Resurrection.
Allaah Knows best.