No matter what you write in the letter to the DA ... they can (and probably will) continue with efforts to prosecute the case. Especially if there were any injuries involved (including bruising) or if there are children present in the home. I would suggest a slightly different tactic to deal with this situation.
He should have a lawyer representing him. That lawyer can present proof to the DA that he is getting anger management counseling, assistance with finding a job, and grief counseling. That, along with a letter from you asking for the charges to be reduced or dropped, might buy him some time to prove himself! He would probably have to continue counseling, make a strong effort to find a job, and stay out of trouble for a few years. If he does all that ... maybe the DA's office agrees to drop the prosecution of the case.