Kate,
I am so very sorry to hear your story. I went through this as well, although not to your extreme. You are wise to really focus on taking care of your other children right now. Your son really can't see another point of view. That's the drugs talking. The most difficult thing for me was truly trying to understand my son's thought process (or lack thereof!). They don't think things all the way through, their needs are immediate -- right now, right now -- is all they know. Until they are clean and sober for quite a while, they don't even have a glimmer about what we are talking about. In the program my son is in now, they pair them up. One of the guys said they do that because "an addict can think of something stupid to do and do it in 5 seconds flat." By having a peer, they learn to look out for each other's stupidness. It also teaches them to think about someone other than themselves.
You have done a lot -- take care of yourself and your other kids. Do you have a support group? Al-anon? Something? Feel free to write privately if you wish.

Hugs and peace in your heart and home.