How old are you? If you're a kid, there is not a lot that you can do, I'm so sorry to say. There is not a lot even an adult can do for someone else with an addiction besides point out where they can go for help. Alcoholics Anonymous for him, Al-Anon for family members, so they can learn how to deal with the fact that he won't help himself. If there really is anything you can do or say that will have an impact, Al-Anon is the place to find out.
If you're just asking about what you might prepare to try to get him to eat, you might try some Lipton's Chicken Soup, the kind that comes in an envelope and you mix it with water. Easy to fix, and even the sickest of people will usually drink it.
I'm very sorry you're in this situation. Please remember that his choices that are harming him, not you nor your ability/ inability to help him. Even if he is making terrible choices, as an adult he has the right to them even if they harm him, no other person really can impact this. It is hard to take if an addicted person has chosen his addiction over life and the people who care about him, but it is not your fault. It's not your responsibility, either. I am so sorry again.