Hi Jasmine
Blended families are hard on everybody. There are so many issues that you raise here that are difficult: that the mother does not share information; that your husband blows off the issues of his son; that you have to take responsibility for a child with problems, where you don't have the same degree of control that do his parents; that your kids and you have got to deal with the result of all of this.
Where to start?
I think that I would start with communicating directly with husband. Except if he is prone to not want to deal with things directly, he may not want to work with you to confront the problem. So there is that.
If that is the case, I would recommend either Al Anon or counseling, for you, and/or marital counselling for the two of you. You would be getting help for all of you: for this child who needs the support of his parents, for you, for your husband and for the kids. That you are feeling as you are, means that the rest of the family is likely feeling it too. Maybe most of all, your poor stepson who may be giving voice to family dynamics of which he is a victim.
You can't control anybody else. But you can increase your own power and ability to advocate for yourself, your kids and this child, so that his parents step up to help him to suffer less and to set limits on how his behavior affects others. Ultimately, you cannot make anybody do the right thing. But we can get strong enough to voice what we need, (and what our kids need too, who are dependent on us as advocates) and to insist that we get it.
I feel compassion for your stepson.
PS Although An Anon is for family members of Alcohoics, in reality anybody can go to open meetings and the process is very helpful for family members who come to this site.
