One of my red bellies lost one eye and most of its fins bitten off and he pulled through. Just keep the water clean and make sure there's no more aggression. But Idk that's a big chunk of body missing
One of my piranhas had a very similar injury(anus completely gone) about a year ago and it healed over completely. In fact i can't even tell which one of them it was anymore. That bite is tiny as far as piranha bites go so i wouldn't worry, it'll heal over.
I have seen worse. He should be fine. Just keep an eye out for infection and treat as necessary. Sometimes isolating them can cause more stress, but if appears weak you will have to.
Steven tht Red Devil looks awesome!! I think he looks exotic lol.