Prison is not working for my son from Pam

Hello Prison Widow Blog. 
I am an avid reader of your blog and I am a mother of a repeat offender who is in prison at this current time. 
Despite my son having a wonderful supportive family, he is back to his old ways almost immediately on release from Her Majesty's Prison. One of the most frustrating things is, he isn't a bad lad. Even the prison officers have commented that he is polite and causes no trouble. 
My son however is a drug user. Our family decided on an intervention because we didn't have any other options left, but since researching and taking note from other families of drug users on a forum, we now know that we as a family cannot help unless my son wants to stop using drugs and goes in to recovery. Other concerns is that he is using drugs in custody although he rarely asks for extra money. As a family, we do know that he does portraits in prison which we are guessing are in exchange for drugs. He is an exceptionally talented artist. 
Should we and of course other families in this predicament reside ourselves to the fact that as long as there are drugs in prison, there will be no such thing as rehabilitation? We are an open minded family and of course theft and house breaking should warrant a prison sentence, but what good is a prison sentence when all he does when he gets released is do it all over again? I fully understand that he needs to be in prison in order to protect the public but he is continuing too re-offend so public protection is only temporary which again I need to stress, what does the purpose of prison serve for those like my son with a drug problem? Love to all, Pam.