Oscar Hanmer was last seen near to Victoria Gardens in the town centre at 9.25pm on Monday, December 16.
He is described as white, 5ft 8in, of slim build with short blond hair.
He was last seen wearing a black North Face puffa jacket and black joggers. He may also have a bicycle, which is a black trek bike with blue writing.
A spokesman for Northamptonshire Police said: "Officers are becoming increasingly concerned about Oscar."
Anyone who has seen him or knows where he is should call Northamptonshire Police on 101, quoting reference number MPN1/3973/19.