I have a 40 year old. They are far from children. They are middle age adults.
I dont know your back story, but if he steals from you, hurts you, drives you insane...get a restraining order against him. Dont say anything you arent sure you can do. Just dont answer his calls or texts or the door if he comes over (see restraining order). Change your locks. Get a security camera. Do not see or talk to him unless he is clean and, even then, because relapse is common, only meet him in a public place, like a coffee shop, not in your house. Ever. If he stole recently, call the police. What a betrayal.
I am assuming he is intolerable. Most addicts are. And your son must have been on this road for a while now at age 40.
You cant be young and you deserve to enjoy the rest of your life and feel safe. Do whatever you have to do to make that happen. Do you have other family or close friends? Church group? Any support?
We are here for you. Most if us are familiar with drugged adult kids.