Your first DUTY is to protect your child at all costs. Put his NEEDS (and his need for safety) first....always. Your husband is too unstable and potentially dangerous for him to be anywhere around you or your son (especially your son). And think of the role model he would be for your son.
If your husband wanted you and your son more than the booze and drugs, he would have checked into detox and then into a treatment center. If he's on probation, treatment has already been offered (or court-ordered). At this time, you can do nothing for him, other than to avoid him. HE must want the help for himself.
He won't see ANYTHING until he's clean and sober. And until then, you cannot afford to allow him into your life (or your son's). The man cannot be any type of father (except a dangerous one who's a bad influence) at this time. Face the facts. Your son can't afford for you to be naieve.
PLEASE work on being independent and taking good care of that boy of yours. You have been blesssed with a child who looks up to you and counts on you to protect him. You are all he has at this time. Working on the relationship w/ your husband should come ONLY after he's clean and sober (and he's STAYING clean and sober). If he is not interested in getting/staying that way, for your safety and for your son's, you need to file divorce papers. Talk w/ a pastor or another trained professional before taking such a big step; it could really help.
God bless you and good luck. Remember, your son is counting on you to protect him while your husband is counting on you to enable him by taking him back now. Be smart, please. I am going to say a prayer for you and I hope you will do the same for you, your son and even your husband.