Mr Sir doesnt fill up Stanley's canteen because he is angry.
Stanley doesn't deserve to have water.
A boy from another tent mocks Mr Sir about the marks on his face which makes him angry. Mr Sir grabs the boy's head and bangs it against the counter. He yells at the boy telling him that there is nothing wrong with his face.
In the earlier chapters, Zero and Stanley make a deal that Stanley will teach Zero how to read and write if Zero digs half of his hole for him.
Mr Sir doesnt give Stanley any water because he blames him for the warden scratching his face.