Personally I would send the letter to him (keeping a copy), if it is returned unopened, or you get no reply, I would then send it to his local newspaper, after all, what have you got to lose at that point?
I'd also send a copy to my local newspapers EDITORIAL PAGE for the publics viewing of what this cult does to and with its children.
This would be a major statement about Jehovah's Witnesses but it would show complete disregard for your dad. If your aim is to irrepairably hurt him, fine, but if you do love him and want him in your life it's a horrific approach. I hate disfellowsipping and shunning and the nameing and shaming that Witnesses engage in and this smacks of an equivalent.
Dawg, I really empathise with your situation but don't think sending such an accusatory letter will make your dad change or make you feel any better. Getting it sent back will frustrate you and continue the cycle. I'm pretty surprised at the vengeful nature of the advice on here. Maybe I'm a sap, but it works well for me.