Compile photographic evidence of him being abused then call social services. It'll be rough living for him for a little while, but he'll be far safer. Then you can try and get custody yourself.
This
Constantly pile up photos, personal accounts, whatever. Every little bit helps. And please don't beat up the guy. It'll make your case harder (use it as a last ditch effort though...shit-head deserves it if he's hitting the poor kid).
If you accuse your mother of being unfit because of mental illness then you'd likely want to get a professional to diagnose her.
Good luck, never give up.