I can't say what I would want do on here, because of the trouble I would get in. I probably wouldn't anyways.
However a more reasonable option that is legal is to get an attorney and a private investigator also a hidden camera on you and microphone, with incriminating shit on it plus any dirt a really good sneaky ass P.I. could possibly dig up might help in a civil court. Also any threats of violence against you by him if he is stupid: get a restraining order and other shit, if their is anyway to fuck up his life, it would be a good legal option. Especially if you got him on state or local crimes, yes threats of physical violence is a crime and also state and federal trouble at the same time is great.
If you could get shit on him and possibly make the case, you could get it reversed and cause him trouble.
I don't know your financial situation or what exactly, except what you wrote; the exact type of scumbag, but if he has stuff in his past and possibly illegal shit going on, that would go a long way towards fucking him over( figuratively speaking), do you live with them, if so check the state laws on hidden cameras in public areas of the house and audio also.
I lived briefly in Maryland in the beltway area. If there are any financial ' irregularities', Dress nicely and with proof of ' tax irregularities' that are in noway associated with you or your mother with an attorney go down their( IRS head quarters, and make an appointment first )and if can get someone to listen; also turning on the water works if you can do it easily might help. Especially if it is a woman or a kind man. That maybe hard, they are IRS agents. So sympathy is a long shot but willful tax 'irregularities' is going to interest them.
Not sure if he is an asswipe or a criminal asswipe. If the later is the case and you can prove it. The feds don't play.
If he is a narcissistic type or a sociopath it will be easier. Also if there is really no dirt on you pertaining to the case, that would be great.
Also things drag on in civil cases, so any legal trouble on him if allowable would be great. Not sure of State laws; however if the business is doing business over state lines, a federal lawsuit could also possibly be used.
You need to make him; with proof of course, that he is a criminal dirtbag.
Not sure because of the unknowns, but legally go after him.