Has he tried plugging into another bass amp to see if the problem is with the amp or with the guitar?
If the guitar is the problem, it is usually the electronics contact within the knob. You will need to bring it to somebody who knows how to do a re-wiring. Mike from Mikemann Studios is particularly good at this sort of work. If not, you can try Guitar Workshop or Guitar Connection.
You can also try bringing back to the shop where you bought it and get them to take a look at it.
If the amp is the problem, bring it back to City Music if there is a warranty and get them to repair or replace it.
Did you not test the bass and amp at the shop before you bought them?
If the problem happen so soon after you bought them, bring it back to the shop and exercise your consumer rights.
