I think it is possible,cause I stopped after using it 3 times a week for about 5 month straight. I stayed clean for about a year and a hlf ,but relapsed again but i do it alot less maybe once every two weeks, but i still got to stop completely. cocaine is very addictive and impulsive, any settings that he may be at or routines that associates when hes using the drug may trigger an intense cravings for it. So my suggestions is first get him to stop first, than find set activities to do everyday etc. like sports (handball was my choice to stay occupied and feel healthy). Goodluck!
It's possible that he can quit on his own, but cocaine is highly addictive so it will probably be pretty hard. Once a week for 5 months isn't as frequent as some cocaine users, but it's certainly enough to make a person addicted. There are things he can do to help stop, like change his environment by avoiding the situations that trigger his use and find healthy hobbies to occupy his time. Your support can also help.
An inpatient rehab program is probably not necessary for him--he could attend behavioral therapy sessions with a counselor to help him kick his addiction without completely derailing his life. Cognitive behavioral therapy has been shown to help treat cocaine addiction, and is sort of a middle ground between quitting completely on your own and entering rehab.
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