I really can't believe that I'm going to forgive him for this.
Why are you forgiving him then?
Because it would be stupid to hold grudge against him.
You're absolutely right. Holding  a grudge will harm your mental health so it's better to forgive people.
Can't I just be angry at him?
You could be, but you'll be happier once you just forgive him. It's not good for you to hold onto anger. You don't have to continue hanging out with him, but for your sake go ahead and forgive him.