Juices contain fruit sugar and vitamins, as well as water. Therefore, juice drinks are appropriate substitutes, especially if a person is water-depleted from strenuous physical activity. Also, because of a high sugar-content, soft drinks are not appropriate for overweight persons. You would do better to slake your thirst with water or juice, and leave sodas for a treat. Unfortunately, he is an alcoholic who actively drinks.
Lymphoma causes few early symptoms.
Of course, this is precisely the time in which treatment will be most effective. Therefore, your father has the greatest chance of cure if he begins therapy as soon as possible, before the tumor has had a chance to spread. Unfortunately, active alcoholics rely on an emotional defense mechanism called denial. Denial is part of the disease of alcoholism. Given the fact that your father is denying his lymphoma, I suggest that you obtain a second or third opinion.