Merlin Ferry, 22, was arrested in London last September after police pulled him over in an uninsured vehicle and discovered the weapon when they searched him.
The Roxy Music star's son appeared at Isleworth Crown Court, on Tuesday and was reprimanded by Judge Paul Dugdale for allegedly complaining about the "inconvenience" of having to get up early for court appearances.
Ferry avoided a prison sentence but was warned he will be jailed for six months if he is caught carrying a blade again.
Judge Dugdale said: "Those that have the chance to make a success of their lives without the normal hardships of life are very lucky. It has taken a bit of time for you to realise that luck and good fortune does not take you out of the normal world or that normal rules apply to you...
"If you carry a bladed article again in public again you will spend six months of your life in prison and the consequences of your lucky life so far will be devastating. Don't think that will not happen, it will."
Ferry previously escaped jail time in 2011 after his lawyer successfully argued that the Gurkha knife he was carrying at the time of his arrest was part of a costume.