Addict parents are erratic and confusing to children, so it is best if a child does not live with an addict in the home. The child does not understand the mood swings, the sudden changes in routine, the tension between the parents, and the feeling of dread that pervades the household as the addict obsesses about the next hit and the spouse swallows her anger and pain. It is best if the addict does not reside in the home.
In your situation you are without any legal protections for your child and for you, and it is so important that specific conditions are set and custody issues very clear (if you die tomorrow, will an addict be raising your son?)
I think a legal separation and supervised weekly visits would be a safe arrangement for now. Your 6 year old son is old enough to learn about his father's addiction but it would be best if that comes from a skilled counselor or social worker. That person can explain the medical condition of addiction in a way the child will understand, and a skilled counselor can also explain why living apart from the father is necessary for now. Families of addiction need outside help. Alone, they are lost. They make many mistakes.
I hope you will seek professional help for the very serious situation you are facing. If your husband does not seek recovery, great crisis lies ahead, and your family needs now to find a counselor you can turn to when and if a tragic event occurs. Pill addicts often end up on heroin. And in prison.