In the movie The Jungle Book, Mowgli is living in the jungle/forest for a long time already (may be 2-3 years) before the dry season. And he is known to most of the animals in that particular area. However, Shere Khan arrives near a water body and threatens the wolf pack to give up Mowgli. And he didn't attack at the time as it was a truce period due to drought and he later comes back after the truce. But why didn't he seek out for Mowgli before the truce period? Was he not aware that the human child lives in the jungle before?
It is plausible that Shere Khan did not know that Mowgli was living with the wolves before the truce. When Khan arrives at Peace Rock he references a new smell (talking about Mowgli), and many of the other animals are shocked to see a mancub in the jungle. Animals probably kept to themselves as much as possible in non-peace times to avoid getting eaten and what not. That said, it's possible that Shere Khan did know he was there either because of the events that unfolded between himself and Mowgli's father in the cave or just because he is a badass and knows what goes on in his jungle, in which case I'm not sure why he wouldn't have killed him.
Shere Kahn has been away from this part of the jungle, and is not aware of the man-cub.
Shift your hunting ground for a few years, and everyone forgets how it all works
This implies that he has only just returned to this area, presumably looking for water due to the drought.