Good job on living your life despite the uncomfortable feeling that comes along with the situation.
Other posters are correct. This absurd behavior simply assures that each and every "worldly" family member will never, ever convert to JWism. Idoits. What a fine witness to Jah your father proves to be! Clap, clap, clap.
I wouldn't make it a mean letter. Simply say you hope he has seen how every family member finds his behavior childish, selfish, and repugnant. Ask if that is how disfellowshipping is supposed to work, making JWs look like idiots. My bet is that he will be the one to duck out of all future reunions because of how it makes him look.