Docking is a fairly controversial issue but if you do a search there are some old threads on it. It is now illegal in England and Wales to dock any dog which is not going to be used for working. There is a list of breeds, including cockers, which the 'working exemption' can apply to provided the criteria set out by DEFRA are met. In Scotland it's now illegal to dock any dog full stop regardless of breed and working status. Working cockers were/are traditionally given a longer dock than show type cockers with only the last third of the tail removed. It was done to prevent tail injuries when they were working in the field. I have 3 working cockers, 2 of whom work and 1 which is being trained and one of them has a full, undocked tail, one has a 'proper' long working dock and the other has a too-short dock for a worker. She is a rescue so I don't know her background or where or why she was docked so short.
I'm not sure why your dog would be undocked whereas his brother is docked short, and also badly by the sounds of it - a working cocker should never be left with just a very short stump. Are they littermates or are they 'brothers' but from different litters?