You won't hesitate to contact social services if there is a serious risk to your child? Your AGF drove drunk with him in the car. Is that not serious enough? It only gets more serious when she actually has a car accident.....pleeeeez don't wait for that.
You are not stuck in this mess. She is your GF, not your wife correct? You can still back out of the house, don't tie yourself to her any more than you are. Take that child and run for the hills. Get him to safety, and protect him. This is a progressive disease. She is already decompensating according to your post. However bad it is now, it's going to get worse. So the fact that it hasn't turned catastrophic "to date" doesn't mean anything. It's not her parents job to protect your son....it's your job. Read on here about the suffering adult children of alcoholics deal with throughout their lives from living with this chaos. He's 2, it's not too late to save him. You can not save her, but you can save him.