D.,
Congratulations for making the hard, but right, choice. I can only imagine how hard that was for you. Keep reminding yourself that you did the adult, difficult thing for yourself, your son and your husband. Continuing to expose a child to drugs was not in anyone's best interest.
The important thing is to listen to your son's pain. Is he worried that this is his fault? That would be so typical. He needs to know that this isn't his fault, that Daddy wants to see him and that he loves him, but that he can't right now. You might want to arrange some visits with daddy if you think he can get himself together for an hour or two in a public place, just to reassure your son that he is OK.
Have Daddy tell him (if you think he can) that he is sorry that he isn't living with him, but he has to work and has to get better because he has been sick. Don't hold out false hope, but see if Dad can reassure him that this is not the boy's fault. If Daddy is going to use this as an opportunity to go off on you and tell him it is all Momma's fault, then don't even consider it. Try written messages or phone message that you can control.
You also might want to join others who are or who have been where you are. Al Anon is for families of alcoholics. Call them, they are free and in the phone book. Narcotics anonymous may have a similar support group.
If your family is supportive, you might want to cut back on your work so you have some physical reserves to deal with the pain and disappointment you are going through. Best of luck. I admire you.