Its a fair but firm letter.. and the idea of telling him he is NOT WELCOME in family therapy will throw him off his throne for sure.. he wont likley say it, but he will be irritated he's not welcome there...
That's just too damn bad...
Start collecting good articles on rebuilding a marriage, print them up and leave them at YOUR desk and work area in a folder...
He will snoop and read them.. Highlight the spots you want him to know ...
Just keep doing that.. its a very subtle way of educating him with him saving face...