It may be cowardly, but maybe not...If you don't think you have the strength to say no, then don't pick up the phone. I did that at times because I felt it was better for me to not engage than to fold, and I did not trust myself not to fold.
It isn't unusual for the first week or so of rehab to be a hell week and then things get much better. I was grateful when my daughter's rehab had "black out" (no contact) duing that first week...Sort of wished it was for two weeks.
Your gut instinct is right - he is right where he should be. Maybe you can get a few Naranon meetings in while he is working on his own recovery. As far as a letter, perhaps for now the best is a nice card with a sentence or two from you just letting him know you support him and care. I know my daughters both looked forward to mail when they were inpatient. I tried to keep it loving and light and leave the heavy stuff and the "I think you shoulds" out of it. That I tried to leave to the counselors. Honestly, it was a relief to just get back into the role of unconditional love and stop trying to control.
All the best to both of you.