I plan on not going to my best friend's wedding.
Why would you not go to their wedding?
I am mad at him.
You should be there for your friend. It is their wedding and you should support them.
I am only going to hurt his feelings if I show up to his wedding and I don't want to ruin his day.
I know you're mad at him and they may be mad at you also, but showing up to the wedding would make them happy. I believe showing up would give you the option to talk with them and fix the issues in your relationship.