I had a similar experience. My son had become an addict.
Here is my advice. Start with the solution and work backwards. Your son needs to get into some kind of recovery program. You need to figure out all the options available and figure out what ones are viable from a financial perspective. It could be AA; or it could be intensive outpatient. There are options for rehab and in some cases, if you are lucky, insurance might pay some of it.
Your son might be in denial. Then again, he may be looking for a way out of the addiction. He won't listen to your nagging, but he might be prepared to consider treatment. Tell him that you know that he is in trouble, but you still love him. Tell him that he is suffering from the disease of addiction but there is hope. Walk him through the options. Tell him that you will support him in his recovery but that you cannot enable his addiction.
Finally, I found that prayer invoking the help of a higher power helped.