Pre-teen step son questions..?

I am the mother of a 10 year old step son (as well as my own 7 and 5 year olds). We've had custody of my step son since he was 1. He's always been a very loving boy, very respectful and kind hearted. His birth mother has not been in the picture much (she drinks and uses drugs). We have always been honest without being too informative with him about her. He knows she makes bad choices - with that said, we've never came right out and said *she's a druggie*. She comes around about once every 2-3 years and for about a week each time. When he was 6, she came to us and said she wanted to get clean and be a mother, so my bright idea, I thought, if we let her stay with us, seeing him daily would help her fight harder to stay clean.. I was wrong! I noticed our things coming up missing.. but still tried to help her. One night, she had asked us for some money. I told her I would gladly go get her whatever she needed (cigarettes, female products, whatever), but I would not give her cash.. My step son was holding on to her (he had this fear if she left, she would not come back).. she informed my husband and I (right in front of our son), that if she does not get the cash, she would *blow her head off in front of her son*.. Of course, I make her leave and that was the last time we allowed them together! My son has dealt with it fairly well. We found out she is dieing.. she only has about 6 months to a year to live. I decided to go over to her and see if she's decided to straighten her crap up before its too late for her and our son.. when I show up, she is overly drunk. she does not even remember his name or how old he is. I cried all night because I want them to know each other.. my biggest fear was coming between them 2. For his safety, I have no other choice! He's never questioned why or what if.. he's a good boy. the past few months, he's changed. He's becoming mouthy, demanding to know how it would be like to live with her. My first thought was to let him see it with his own eyes, but I can't.. that would damage him worse than being mad at me for not allowing him to see it. He's stopped doing his chores and started taking things without asking.... its not him! I know step children go through the faze of hating their step parent, but how do I help him?? I caught him kicking his 5 yr old sister - out of anger, I've caught him abusing our dogs.. he loves his dogs and his sister. He said he kicked his sister because he cant see his sisters from his mother (out of my control... they all live with their fathers and we've tried everything to get them all together when we can.. but they wont).. I am almost to the point of getting rid of the dogs for their safety... which is not fair to our other 2 children! I want to help my step son.. Any and all suggestions are welcome!