However, the form is not perfectly regular. The general regularity of the meter and rhyme contrasts with the unconventional nature of the thought expressed in the poem. He doesnt have self esteem - he sticks strictly to Christian Doctrine. The clouds are our physical bodies. They are imperfect.
Clouds are also weak and insubstantial, just as bodies are in a traditional Christian sense.
To earn Heaven our physical bodies have to suffer. If on earth we bear the suffering, then in Heaven we will rejoice like lambs. She uses good metaphors. However, the boy has a somewhat innocent interpretation of the mother's words. The boy colors the clouds - he makes them black and white.