Answer

As he climbs further up the house his mother is still holding one end of the tape measure, whilst he keeps hold of his end. She keeps hold of it as long as she can, not wanting to let go, just as she doesn’t want to let her son go and be free.

This is why he opens the latch of the window or skylight so he can escape into the sky, away from his mother.

The distance will be so far that she can’t keep hold of the tape measure any more.