Just my opinion, don't send the letter you wrote. It may not help and look like you are trying to guilt him out of a D. Tuck it away...I had tons of letters I wrote to H and I finally did give him one...now I know that H read it but it made no difference to him at the time...H said it sounded like I was desparate and it turned him farther away.
As far as the letter from S9. Let him decide what to do with it without any influence from you. It is so difficult for kids and getting his feelings out to you may have provided some relief. If S9 is ready to give the letter to his father then let him do so. I would just feel awful if H did not react to it the way S9 hopes, that may provide more hurt.
