You're right. Speaking to him about his behavior, giving him the letter, drawing him a picture--none of it will change anything. Zero, zip, nada.
It's not HIM that needs convincing, it's you. If his behavior is unacceptable, why are you accepting it? He doesn't need to know your thoughts, you do. What are they? You say you are not ready to walk away. Do you know why? Are you hoping if you say just the right thing, write the perfect letter, he will slap his forehead and say "of course! you are right! what was I thinking?" (I say that because that's where I was for a long time.
)
Your mental energy is much better spent figuring out you than him.
L