He is not going to sort out his alcohol problem for the sake of his son.
He is not going to sort out his alcohol problem for anyone, including you.
His primary relationship is with alcohol. You and his son and his parents come a very distant second.
End this relationship. Tell him if he wants co tract with his son he will have to take you to family court and get an order.
Allow the GPs to keep up the relationship
But all of you need to start modeling better responses to the toxic person in your lives, the alcoholic.
You all need to join Al Anon for Families. I can't link because I'm on my phone. There's information online and I really urge all of you to start relearning your responses to abuse.
You need therapy too.
You need to firmly believe that you didn't cause the problem, you can't control it, nor can you cure it - and you need to wash your hands of it.
Ditch the toxic optimism that has robbed you of four years of your precious life. This toxic optimism (aka pride) is the lingering legacy from your childhood. You have to face it and become very conscious for the rest of your life of where it tries to lead you.
I highly recommend a series of YouTube videos by an Australian therapist - Vital Mind is the YouTube name iirc. In particular, look at his Empath videos. I'll post one later when I'm home from work.