Drug addict on release from prison? - From P

Hi to all. I have just read a ladies story on your Twitter feed and would like to comment.
My son is also incarcerated and is a drug addict.
He is also still using whilst he is incarcerated but the biggest weight off my shoulders is that I know where he is. When he was in the free world; I was always anxiously waiting for the phone to ring or the police knocking on my door to tell me that my son is dead from an overdose.
The guilt I feel saying that I am thankful that my son is in prison tears me apart.
What are the prisons doing to combat this epidemic? My son is released in 2022 and I do not want him to be released from prison still actively using drugs because what was the point in sending him to prison in the first place? Where is the rehabilitation? How is the system working?