my 2 uaru had little stumps left when i got them at 1" and now they have fully grown tails at 5" with no scarring. the tails took a while to grow back , about 2 months with melafix, and i think people under estimate the resiliance of a young fish. just like our own children they often bounce back with a full recovery.
try melafix and w/c twice a week for a couple of months and it should be fine unless he is an old fish or like the others say the damage is extreme.