Other than bringing in professional help - if it's lawful in your state, record all conversations with him (digital voice recorders, smart phone voice recorders, and if necessary video - but this can antagonise people further).
If you aren't allowed to use those methods (and even if you are) - ask to only communicate with you through writing - either text, or email. This of course does make some communication difficult, but on the very positive side, it allows you to have evidence of what you have talked about, and it usually means they won't be abusive (in writing).
Keep a diary. Don't make it about petty things, and don't try and guess his motivations in your diary - try and keep what is written objective. That's because if you use it as evidence, defence will pick apart a diary (if it's presented as evidence), attempting to paint the writer as petty & vindictive...anything to not look at the content...so the content has to be objective. How you felt can factual, what was done can be factual, but why he did what he did, and the intended consequences of his actions - are just a guess on your part (no matter how well you know him)