IMO, I would not send any letters. Write them, sure. Keep them as a journal type of purging or burn them or whatever, but I would not send them.
As far as the moving on part & whether he understands your meaning... in all honesty, I don't see why it matters.
I think in your prior threads you mentioned that he does AA - hopefully he has found a sponsor. That's who he needs to be talking this stuff over with, that's who can help him understand whatever may be causing confusion. JMHO but the longer you spend {even minutes or moments} worried about all this stuff from his perspective the more crazy-making it becomes.