The drinking is what caused the stomache issues in the first place, and of course is a depressent. In addition,
alcohol should not be mixed
with the medications he is on. In a small way he may be drinking for physical reasons, such as pain, which is why I think relief of pain will also cause a change in attidue in your father, and lessen the need to drink. From there, you may suggest to your father seeing a dietician and/or physical therapist to talk about health eating habbits and weight loss.