What are the chances of someone with a severe addiction recovering?

My ex boyfriend is now incarcerated for 5 years. He commited a felony while under the influence of drugs. He has a severe addiction and is in his forties. Although he has tried many times to stay away from drugs, he has not succeded. He has destroyed his life, and finally hit rock bottom. While on drugs he is a disgusting person and has become abusive. When he is straight he is a wonderful guy! Not knowing what the future will hold for him, what are the chances of a lengthy incarceration straightening him out and him staying away from drugs once he is out? Also, I have been advised that abusers never change and just because he stays away from the drugs doesn't mean that he will never become physically abusive.