I'm going to tell my friend that he is the most toxic person I know
What did he do to you that was toxic?
He keeps mocking me for being fat.
It sounds like your friend is insensitive for mocking you. It is fine to let somebody know when they have hurt you.
I'm going to pummel him with insults about how fat he is and how he shouldn't mock me.
Neither of you are going to come out of this situation well if you are both determined to insult each other. Is there other tension between you that may be making things worse? Because doing exactly what you are accusing him off isn't going to solve anything.