All for naught, clo. One who is baptised Catholic cannot write a letter to leave the Church.
Baptism puts an "indelible mark upon the soul." It can no more be removed with a letter than could a tattoo.
Baptism cannot be undone. Not a letter, nor excommunication, nor joining another church, nor being baptised under another church, nor renouncing Jesus, nor embracing Satan, can stop a Catholic from being a Catholic.
Once baptised, you're a Catholic even if you support abortion. And even if you've HAD one.
You're just not a GOOD Catholic.
What your letter-writer WILL probably accomplish is getting himself excommunicated - not that he'll much care. This is probably the case with OCU too - and, unfortunately for him, he probably DOES care.
It's a pretty neat racket the Catholic Church has, when you think about it.