The American Cancer Society says that esophageal cancer's survival rates are "18% of white patients and 11% of African-American patients survive at least 5 years after diagnosis. These figures take
into account all patients with esophageal cancer, no matter what stage they were in at diagnosis. Survival rates for early stage disease are higher."
This means that most patients (82% white or 89% black) will not live past 5 years. You'll want to help him on a daily basis, but make sure you've had some long-term, end-of-life discussions, so that when that time comes, you will know what he'd like to do.
You mention that he is suffering badly now, and I can understand why. He's had major treatments in a relatively short period of time. It seems like his oncologist is aggressively managing in the best way for his condition, which is very good.
While smoking has been proven harmful to the body -- and most health professionals advise against it -- there is little definitive research of the impact of smoking on chemo on esophageal cancer. In lung and other cancers, it has been shown to decrease the effectiveness of the chemotherapy agents and decrease overall healing in tissue.
Although you may not like that he's smoking again, he may do this for some relief, enjoyment or distraction. While you cannot make him stop, let him know that he's acting contrary to his treatment goals. Understand if he needs to make the decision to continue doing so.