When I saw the title of this thread I thought the "demon" you were speaking of was alcohol, then I read the opening post and now I wonder if the "demon" you speak of is the mother of your child!
Can you "beat" the demon of alcohol? Probably not, BUT...it's not really alcohol you are fighting. You can walk away from it and let other people fight it if they care to, it's not your fight. What you want to beat is your own dependency on it. And that, for sure you can deal with.
I DO have experience with a spouse (now ex spouse) who wanted to see me go under, because it would have been very convenient for him if I were to disappear. But I had other plans for myself.
Get sober, stay sober and be an awesome dad. Her choices and priorities are hers, try not to get caught up in them or judge them. You stay busy being a great dad to your son. Being involved and encouraging. Let those teachers know they can call on you, and get involved in the positive things at his school and in his life as well. Don't just show up when there is a problem, but see if you can volunteer, make sure you never miss an opportunity to chaperone a field trip, or participate in an activity. THAT will be a huge impact on how your son feels about school and will help with how YOU feel as a parent.
I didn't want to be an "everytime I messed up my mom got involved" sort of parent. I wanted to be "everytime something neat was going on, my mom was involved" sort of parent. Associated with good times, hard times, dull times, great times. I wanted my kids to know I was their parent, no if's and's or but's about it.