There is most certainly a reason to get him to another city or State. If you want it to be sucessful it is best to go far away from friends, and dealers and others that you used and knew in your past Addiction life! The better the chances of Recovery to work, if you have lots of time away from people you know that use. You need to start a new lifestyle for it to work sucessfully. Try and do your best, and what a good cousin you are for helping.
I think it probably depends on the kind of support structure your cousin has in San Diego. Obviously he is lucky to have a really supportive cousin who lives close, but does he have other supportive family and friends in the area who don't use drugs? If he has a lot of support in the area he might do fine staying close.
If it's going to be really difficult for him to separate himself from his drug-using environment, it may be best to go to another city or state for treatment. For most drug users it's really hard to quit when they're surrounded by the same triggers and people. Getting farther away often helps an addict to focus on recovery and learning new habits.
So a local center might be okay, but there are definitely reasons why going elsewhere might be a good idea.
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