just tell him if he continues drinking he risks getting cirrhosis of the liver which is not pretty and usually fatal. And make sure you tell him you couldn't stand to be without him. The choice will be his. But be aware that alcoholism is a disease. Most of the drinkers know it is bad for them, the same as cigarettes, but both can be addicting. I think AA could be of assistance. It is free I believe. It would be nice if you supported him by offering to go to the meeting with him. You may learn a few things too.
You don't need to go to a doctor or counsellor to get help. There are a lot of resources available for friends and family members who can't get their addicted loved ones to get help. [link] and [link] both provide online group sessions, so that friends and families of alcoholic can share tips and information. It's completely free, and since it's online your parents don't need to know about it. [link] The National Council on Alcoholism and Drug Dependence has a very good hotline and a lot of connections. They're good at helping family members connect with highly qualified advisors so that you can get strong, personalised advice on how to intervene even if you can't travel or pay money. And of course, if you have a school councillor, they're required by law to have information on alcohol and drug addition resources in your area, and to go through training on how to help kids your age deal with alcoholic family members.