It looks and sounds like a condition called tail walking. It's from a bacteria that's usually caused by an ammonia spike. The spike doesn't give all the fish the disease, but the bacteria is contagious.
Tail walking can be cured with a strong broad spectrum antibiotic. But you need to separate him from the others fast, to protect them from the bacteria. And start treating him asap with the antibiotic. Success depends on a lot of things. How long he's been this way. How soon he gets treatment. Keeping the ammonia at 0 at all times even if that means doing PWC 3 times a day and using a stress coat. How good was his immune system before he got sick and how weak is he now.
Good luck