The hunt was launched earlier this month after the boy, Dawid Żukowski, was reported missing by his family. His father was reported missing at the same time.
The father later took his own life by throwing himself under a train, state news agency PAP cited police as saying.
The body of a child was found on Saturday by a road between Warsaw and Grodzisk Mazowiecki, south-west of the capital.
(pk)
Source: PAP