It's a tough situation and I've been there myself with a slightly different one.
Hubby has a brother who is a drug addict and alcoholic, basically, he's scum and I have nothing to do with him. He was with this girl and had 2 kids with her. She was no better, although beneath the drugs, she was a good person.
Their kids got taken away from them and placed in foster care. My husband and I went to court to fight for temporary custody so we can have the 2 girls live with us. They were 2 and 3 at that time and my own kids were 10 and 7. We won our case so they came to live with us.
His visits were supervised by us all the time and he couldn't just show up at the house, he had to call ahead of time and make that plan to visit. Everytime he came, all the progress I'd made with the girls went down the toilet.
Their mother could only visit the kids at the CAS building. I had to take the girls there and wait for them until their visit was done. The kids were great after seeing her, no problems.
Could the father of your son not have supervised visits with anyone in authority? Does he have any custody or visitation rights at all legally? If not, perhaps taking him to court for child support and visitation could bring up these issues and he'd be forced to have the visits through CAS and he'd have to be sober or they wouldn't allow him his visit.
It's a tough call though ya know? Keeping your son from his drug abusing father to keep your son safe from harm or letting your son visit with him so your son doesn't hold that against you in later years. Either way could prove to be harmful. wow! what a tough place to be.
