Cynical one asked the question I was about to ask also.
I have been around addiction a very long time, in real life and here at SR, and I think I would take one of two paths with this...
One would be to simply say "he struggles with addiction and needs help, help he will have to find himself"...they probably already know this anyway.
The second would be to say "his issues at work are between you and him and you will need to do whatever is best for your company, his issues at home will be between him and me and I am not comfortable discussing them."
This really is his problem to deal with at the workplace. You don't need to cover for him but you also don't need to offer up information that should come from him.
I think it is quite likely he will lose his job very soon, no matter what you do. It won't be your fault, it is HIS fault for using drugs and maybe a good consequence (like losing his job) will open his eyes.
Hugs