I can speak to this directly

I recently had to put my 14 year old painted turtle
to sleep due to a large male RES. They were raised
together for 8 years. He too began picking on all of
my turtles relentlessly. He ripped open his tail
so badly that he got a systemic infection, He was
hospitalized for several days and was given injectable
antibiotics for a week. He was not a candidate for
surgery thus my only alternative was euthanasia.

You have two options, find a new home for him immediately
or keep him in a separate enclosure. Don't let it go.

Linda