There's been some brilliant practical suggestions and sensible advice suggested already, but just to add my 2c/2p ...
Mental health issues and an addictive personality often goes hand-in-hand, with the addiction being a symptom of the mental health disorder. Treating the addiction without treating the underlying cause may trigger further addictions with different substances, foods, gambling or certain behavioral patterns. Mental health disorders are usually caused by chemical imbalances/deficiencies in the brain (serotonin, noradrenaline and dopamine) and substances such as alcohol and cannabis will temporarily cause those chemicals to rise. Other warning signs are consuming high quantities of dairy products - particularly cheese - as these raise dopamine levels, excessive caffeine consumption to combat periods of insomnia and any habitual behaviour that stimulates the brain's reward centre, such as always buying a lotto scratchcard at the store when out to get milk, frequent one-night stands etc.
I have a relative who suffers from schizophrenia and this person abuses alcohol simply to function. From my own teenage years I realised I too had an addictive personality which is why I've always refused to experiment with powders and pills like my peers have and to this day have never set foot in a casino. I suspect that a genetic pre-disposition to mental health disorders was one factor in me developing Post Traumatic Stress Disorder. My ex-husband is bi-polar but for the duration of our relationship refused any medical assistance. Prior to meeting me he'd been an alcoholic but as he was entirely unmedicated and could no longer use alcohol as a crutch, he eventually turned to weed to self-medicate. Whilst it calmed him enormously and prevented the insomniac episodes, he was spending around £50 a week on the stuff and was smoking it in the morning on the way to work and on 'cigarette' breaks at work.
Having lost his job and his home, your son is already at rock bottom. An intervention or parental accompaniment to see his doctor can be considered 'tough love' if he won't go by himself. It won't be easy but as he has already opened up to you, his future seems very hopeful and he has a brilliant mother who cares for him! All the very best my dear. I'm sure you both can get through this and come out the other side stronger!