It doesn't matter what your ex says... This is about you and your son. Your ex is just quacking like an addict. Trying to manipulate you into doing what he wants.
I believe my son could have a genetic predisposition to addiction. There are addicts/alcoholics in my family and in his fathers family. I'm not going to make it worse by letting drug dealers and active addicts in his life. That might give him the impression that it's ok to use drugs because mommy. Then he would have both the genetic predisposition AND the environmental factors influencing him. It would make things twice as bad.
Take it a day at a time. You don't have to solve all these dilemnas right now. All you have to do is take care of your baby to the best of your ability and make WISE decisions based on what is happening right now. EVERYTHING else will work itself out in time. Including your ex's addiction. Believe it Merrygoround.
You can focus on trying to save your ex or focus on your protecting and raising your son. It's your choice.