Be real with him, he won't want to here the random bullshit that comes with it so "soften the blow", just be straight up. Remind him there are people in his life that are there for him and if you want to take an extra step, offer to help him in personal affairs. The reason he drinks is because he can't fix them or control them, so offer help.
Be honest with him, but don't judge him for his actions, let him know about the awaiting consequences if he keeps abusing alcohol. Alcohol ruins life, and doesn't help you to deal with your life problems, it's not the way out of any situation, and it isn't an excuse.