Missmac,
I am sorry that you are going through this, I can feel your pain and hurt and sadness in your words. I think that you have the right to feel the way you do. He loved him once and now he is not the man that he once was.
Unfortunately, he has left you with a lot to deal with and being a single parent is no easy feat ( I have been one since my son was 8) and having all the extra stresses that he brings can not be helping you. I think what makes us sad is that we loved them so much once, we had these hopes and dreams and plans for the lives we would live together and that in itself is a disappointment and makes us mad.
Hopefully for his sake and your child's he will go to rehab and be able to get clean but he has to want it and that is something only he can control. I would guess that deep down he really does want to quit so he can be a better dad and he probably really means it, but this addiction is sometimes greater than their love for anything.
Take one day at a time and focus on you and your son. When you feel angry or hurt at your ex, look at your son and see that together you created a beautiful child. Cuddle up with him and hug him tight, that always used to help me.
You are doing a great job! Single mom, going to school! That is amazing.