1. You can apply to Court for a "set aside" order. The judge only has to grant this if convinced the outcome of the case would have been very likely to be different had you known about it before the CCJ was granted. If granted, set aside means the CCJ does not exist & never has.
2. The main reason you can give for wanting set aside is your brother's employment problem - i.e. that he would have made very certain the debt was paid when the Court claim was issued as he knew he could not have a CCJ. Of course, this will not work unless he/you actually can pay off the debt. I do not know whether you should pay it before applying for an order (you need advice on this either from the Court office or from an experienced money adviser) - it may be that the Court would not hear an application if you had already paid - but if not you must be able to demonstrate the means to pay it immediately an order is granted.
3. The legal position is - as with any joint debt - that you & your brother are both jointly & severally liable for the full amount of the rent. This is why he has a CCJ as well as you.