adorable…
A suggestion.
Now that he has, B”H, quit, make a point of telling him how very much it means to you that he did quit. Make a point of telling him (even if you already did, in your letter) how fearful you were about losing him, about how much you love and need him to be around.
Hopefully it will make him think (at least) twice before he thinks about relapsing to his old ways.
edited