As a kid who was never told the truth, was never given the vocabulary or shown the respect or common decency by my mother (bless her, she was struggling) to be straight with me or any of my sibs I beg you to start naming what is going on with simple accurate language.
If someone, ANYONE, any level headed adult had just once put their arm around my shoulder and told me "this is alcoholism, you didn't cause it, it doesn't mean you can't love your dad, or that he doesn't love you, it just means he is caught in a world where alcohol makes him sick but he can't stop drinking it. It's an addiction. And I am always here for you because you may have many questions, or you may get scared, and that's normal."
I often wonder if my mom or someone had been capable of honesty and providing honest support if my 3 bros wouldn't have ended up alcoholic. I'll never know. :-(
Keep it simple. But remember kids always imagine worse things than reality... and I knew something was wrong in my family as early as 1st grade so what is that 6 or 7? This is a serious conversation, like the ones we have with our kids about sex, safe sex, and drugs and alcohol, that ideally you will have many times over the course of decades as he grows and matures and his perspective changes. You can't start too soon.
Bravo for taking care of you and your son first - you are rare and brave, and it will make a big difference!
(((hugs)))
B