Bruno was sure that he had never seen a skinnier or sadder boy in his life but decided that he had better talk to him. (10.537)
We suppose that when you've been forced to move with your family to the middle of nowhere in a different country, and there are no signs of life anywhere, exceptions must be made. When considering the options Bruno has (Gretel or Kotler), the sad, skinny boy seems like a nice alternative to us!