I personally do NOT feel is so important as to what drugs he is on - but rather WHY he is taking them - what inner pain or memory is he trying to numb...... one must first solve the emotional issue behind the act before they can stop.
I have helped friends through 12 step programs. The drugs and or alcohol they are abusing is important in so far as treatment. An alcoholic can have any internal problem solved but will continue to drink. What he is on needs to be addresses to because some things are easier to break from then others. You are right for the problem to be solved he'll have to deal with what got him there in the first place.
Can you share a little more information about your husband and his history with drugs/alcohol? Did he start abusing drugs recently, or has this been going on since you have known him? Was he using like his when you met him or has been getting progressively worse? Has he ever tried to stop? Has he had consequences for his behavior yet? Like has he missed work because he was to hung over to or has he missed family functions because of the drugs. I am sorry that I am asking so many question but I am trying to figure out how to help you and I need little more information.
From personally experience I know that a lot of substance abuse can come from the desire to cover up problems in a relationship. It is entirely possible that the real problem is something in your relationship, that once remedied will result in a decreased desire to use. I know for me that my relationship drove me to use many times.. Simply because I was not getting anything from my wife.
Drugs can cause alot of problems in a relationship. My concern is, is he around the kids and are they in the home? If so, then I would get out of that ASAP or YOU could get into trouble if CPS actually did their job and something was investigated. I don't know your whole situation, can you ellaborate?
Perhaps he finds being at home boring / scary / any number of other things and would rather be elsewhere. That's not a criticism of you, more an indication of an immature man. The drugs themselves may not be the problem, just something that he does when he's with his mates / not with you. Has he always done drugs, or has it increased since the child has arrived?