You already quarantined him, did you add a cycled filter? That's the most important part when treating a stressed fish
(Torn fins are common signs of stress or bad water quality)
Try feeding him extra nutritious foods, like vegetables and live foods, and monitor the water quality daily, change out a little bit every now and then if the tests tell you to. That's really all there is to do about it.
I bet he'll be alright!
Keep us posted, and good luck!