well knowledge is the strongest form of medication, but change needs to come from with-in.
first of all booze AND weed is a depressent. weed also has highs and psychotic effects but that doesnt mean there isnt depressant with it. unless its medical (which has a much lower amount of thc) it won't be good for him. with weed not being regulated you do NOT know what comes on your pot. including other forms of drugs. laced with coke for example.

i took me two years of sitting in my moms basement every day until she just gave me a good dose or two of tough love. threatened that if i didnt start doing SOMETHING. she would kick me out of the house every day when she went to work with out my keys. and i would have to find something and somewhere to go. at that point i was so appauled at her i just moved out, but literally in the end that started my path to recovery and being functional again.

look for community support groups as an option for him and if he doesnt feel like going, dont give him much of a choice, even if it means dropping him off thereand just leaving, it will make him have to do something involving the world ya know? tell him you will be back in an hour or how ever long you think fit.

even just getting outside, go for walks with him will give him the power of the sun and good weather, which from my personal experience helps EVERYONE with mental illness ALOT.

hopefully he can start seeing pro's and con's to his diagnosis and learn to embrace the pro's. (for example, i use to be quite and shy with anxiety, but my anxiety morphed into being scared of NOT talking which has turned me into a much better communicator which can help a ton in many ways)

even just encourageing him to journal. some sort of schedule can really help some people.

its finding that hard line of not wanting to push to hard, then getting frustrated that its not working, and then you end up having to give tough love.

i hope this helps. feel free to PM me if you have any questions
Goodluck.