There is so little you can do to help your son right now. Al-anon or Narc-anon meetings are for family members of people with substance use disorders. Also, NAMI Family and Friends support groups for people whose family members have mental illness, including dual diagnosis (mental illness and substance use disorder together).
These meetings will not help your son directly, but might help you. Also, the best places to get information about what is available for your son in your area are people who have gone through similar difficulties. NAMI especially might have members and participants who know what resources to try for in your city, county, and state.
What I'm saying is no one can control another person's drug problem. Yes, he owes people money. Yes, he might lose housing. These are not problems you can solve without making the drug problem worse.
Look around your city for resources such as homeless shelters and other supports he can access for free until he is clean and able to be part of the family again.
As you know, mental illness cannot be successfully treated while a person has active substance use disorder. He needs dual diagnosis treatment, inpatient most likely. Like you said, the system won't let him in to the treatment he needs right now. But hopefully that will happen sometime in the near future.