What Can I Do to Help Someone Overcome Drug Abuse?

My Ex boyfriend has a problem with drug use. And we broke up because of his drug use. I care about him alot, and i want him to stop using drugs, because they are bad for his health. He also has problems at home with his family, she isnt the most supportive person in the world.. he was in rehab about 6 months ago, but that didnt help him apperently because he is back in rehab now, but we are starting to talk, and i want to talk to him about his drug use but i dont know what to tell him to get him to quit.

please help me. any advice will be greatly appreciate
Your advice could really help me save his life