Wow - "bathsalts" are tricky because they can be purchased legally most of the time, but can be made up of many different chemicals that have different effects on the body. I'm sorry to hear that your son is hooked on these and I wish they were regulated somehow...that said, it sounds like there are a number of things going on here. First off, it sounds like he is self-medicating his ADHD and Bipolar Disorder (also called Manic Depression). Without proper medication, Bipolar Disorder can escalate into psychotic symptoms including paranoia and violence. Second, it sound like he is involved in a very contentious relationship that is triggering some of his symptoms. This is causing him to become more self-destructive and hopeless.
What it sounds like he needs right now is some external structure before he hurts himself or someone else. Is there a hospital or detox center that you can check him into nearby?