First of all, marijuana isn't physically addicting so forget the traditional drug rehab approach. More likely he was using it to self medicate for unresolved emotional issues, mental illness (like bipolar) or some other mental/pshychological issues.

The pot probably actually helped dull to some extent the emotional pain which is why he kept coming back to it. But since pot doesn't exactly help you become a more productive and socially functioning person, it wasn't a sustainable solution. Pharmaceuticals might get him to the point of being more socially functioning quicker - but has nasty side effects that long term won't be sustainable either.

If the root of the problem is unresolved emotional problems, EFT can really help and could make for miracles - it certainly has for me. Have him try this immediately, even start yourself for support. And all you need is the free manual at emofree.com.

Next pursue natural mood enhances like sam-e, folic acid, (and many others suggested here) etc to help stabilize his mood. The best outcome possible would for him to become a curezone junkie and get into the cleanses and other health protocols that will make him feel physically better than he has in years, and clear out lots of the emotional garbage along the way. The Colon Cleanse and Liver Flush in particular could be life changing, and turn him into a health fanatic like the rest of us around here.