Hi welcome to Fishlore, If I haven't said that already.
I'm sorry your little guy isn't feeling well. It was a good idea to separate him.
Are the barbs new to the tank?
What are the readings for
ammonia, nitrites and nitrates?
In your post you say the other barbs aren't nipping him that bad, does that mean they're nipping him a little?
You'd be surprised what can happen over night when the lights are out.
I agree with freshwaterguy, Stress Coat will help as will keeping his water pristine to prevent infection. Vita-Chem and Fish Protector will help him, but I think you'd have to order them on line.
Is your filter intake that strong? You can try putting a piece of panty hose over the intake. But if his fins have gotten sucked into it, he was probably already weakened by something else.
Good luck, I hope he feels better soon.