My Son Is in a Drug Rehab. I Have Some Questions, Please.?

My son is in a Drug and Alcohol Rehab and has been for 3 months. To work the program, he is looking at 18 months to complete it. So far he is doing good, but often gets depressed. He was addicted to Crack. Crystal Meth and Alcohol. I am not at all familiar with the drugs. If he completes the 18 months, what is the likelihood that he will go back on drugs and what are the long term side effects on his mind and body? He was using for 8 years. And I haven’t seen him in 8 years because of this. We talk on the phone and I send him what he needs until he is own his own. Can someone enlighten me on the chance of getting my ole son back to normal? Thanks so much!
I forgot to mention that my son is 32.
Where my son is, they will not give him any medications. They do not believe in it.