I am pretty sure that you cannot be just anyone and be a payee. They do some checks. I know if there is no family they have people who they appoint. I do know they check for a police record for the payee. They also want to know what the relationship between the SSI recipient and the payee is. "He is my drug dealer" is not a relationship that the government accepts. Well, not for anything other than criminal charges or witness statements in a court case! I am sure your son would not be the first person to try to get his drug dealer to be his payee. If he does end up getting his dealer to be his payee, and you find out about it, you can always call and report this to either Social Security or the police.
Right now, you have to protect yourself. When he is after his money to get drugs, you are not dealing with your son. You are dealing with the drugs. They have taken over your son and they will hurt you if it will get what they want. If he gets clean at some point, he will hate that. Protect both of you from any consequences of that. Get yourself as far away from his money as possible. If you are not off of his SSI account by Monday, take some time off of work and go down to the Social Security office to make it happen. Do it for both of you.
If you don't go to AlAnon or Narcotics Anonymous Family Meetings, please give them a try. Not just one meeting, try different meetings at different times until you find one that is right for you. Each one has a different dynamic. Also consider looking for a private therapist who has experience with substance abuse. If cost is a problem, consider asking if they charge on a sliding scale. Many therapists offer this if you ask for it. You can also find therapists at university clinics by contacting the psychology department of universities in your area. Most of these charge far less than private clinics.