Prison is serving no purpose - from a mother of a prisoner

Hello. I don't want my name to be published thanks.
My son is in prison and is addicted to Spice. He was addicted to it and other drugs before he went to prison. As terrible as this might sound; I felt relieved when my son went to prison because I thought it would take the pressure off me. He was stealing money; selling anything he could get his hands on and manipulating me for money; for drugs. It made me ill and I still am ill over all this.
I thought prison would sort him out but he is still the same person as he was before he went in. I have had to change my number but he writes and sends me messages through other family members and the messages are always to send him money.
I don't know what to do because prison is serving no purpose at all. Thanks for listening to me.