Having the psychological defense of denial is not uncommon. People need time to adjust to an injustice and so please be gentle with your roommate. If, however, this has been going on for months and you see that his denied anger is affecting his well-being, you might want to focus on his inner pain, not forgiveness yet. If he can see his own pain, then he might be able to get at least a psychological glimpse of his anger. At this point, then your slowly introducing the idea of forgiveness might be helpful for him.
For additional information, see: What is Forgiveness?