I have known him since he was knee-high to a grasshopper.
knee-high to a grasshopper
idiomshort; especially relating to when the subject was a small child
The old man laughed, remembering when his daughter was only knee-high to a grasshopper and loved playing in the garden.
Returning to the village after decades away, he found it difficult to recognize the adults who were merely knee-high to a grasshopper when he last walked these streets.