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. Although the pressure of proof may not change his mind, you will know that you are doing everything you can to help him. However, if the doctor is mistaken, another opinion would be invaluable. As you suspect, you have no control over your father's attitudes and behavior. The doctors agree that you have a condition that can be successfully treated.