Question

why does mr. lamb say , so you are not lost, are you ? not altogether?

Answer 1

In this quote, Mr. Lamb says this to Derry because he finds positivity, hope and a flicker of light in Derry's attitude towards nature.

He enjoys Mr.Lamb's garden and loves the rain and all of these show that he still has some light left in his soul and has not completely given up on life.

The fact that he is not completely lost into the negativity of his mind but is simply scattered and needs to be guided to the light is what Mr. Lamb talks about here.