Well, not too up on current issues like this, but technically yes, since he adopted you he became a legal guardian. So he is legally binding to pay. BUT since you are over 18, I dont see why he would have to pay. Like if you were only 17, then he would have to pay all owed childsupport.

Side note. Not bein negative or trying to suggest anything, but if he was abusive why not just forget about him all together. Unless yall still have somewhat a relationshp. Jus my opinion and feel free to tell me to shut up. No worries. Can't go wrong with talkin to an attorney about this though, in civil cases go with one of those he only collects if you do typeadeals. You may not get the whole 20k but you better believe he'd work his ass off to get you a majority of it.