Sorry to hear your mom is so sick. (((hugs to you))). I hope that mailing the letter made you feel better. Whatever your son's reaction to the letter (or lack of), it's very difficult to accept that it's not about us (the loved ones and family members). It's about the addiction. He's addicted to drugs. He's just doing what addicts do.
(((hugs)))
I know it's completely FRUSTRATING.