Regardless of the details, there is a way for a father to see his son: SUPERVISED VISITATION.
Both father and child deserve a chance to have a relationship.
Look, you PICKED this guy to have sex with. You CHOSE this man to be the father of your child. Now, he is yours to deal with, so do what you can so father and son can have a relationship. Trust me, if that man emotionally hurts his son, his son will alienate him and be done with it. But, what if, what if, the father and son forge a decent relationship and have a good life to share? Why would you not hope and pray for such a wonderful bond?
The father has been raised with pill poppers. He knows no different and is dealing with the consequences of his actions. I have known two people who are on the methadone program and they function perfectly normal and have good jobs now. So, seeking treatment does help many people. Please educate yourself about the program.
Perhaps you can have your lawyer order supervised visitation along with random drug testing so you can monitor that he is above board and also have the piece of mind that his visits with his son are the best they can be.
Try to work WITH him, not AGAINST him. Yes, I understand completely that you want to put his son first, I get that. I am a mother myself, so I KNOW how protective a mom can be. But, this guy is the father and deserves a chance to be one. He does.
Sooo, he effed up. But now he is trying hard to get his life together. Give him a chance, please. I am not saying drop his son off for a weekend. As a matter of fact, I would arrange for him and his son to meet away from his parent's house as a neutral environment is probably the best idea at this point. Give that man something to stay clean for, let him see his son under controlled circumstances. They both deserve to know one another.