Can anyone identify or point towards a cause of this visual defect?

A six-year-old child I know of sees heads as though they're shrinking. My friend, who has regular contact with him brushed it off as an overactive imagination, although she did admit it is very out-of-character for him. He regularly comments on the shrinking appearance. The reason I'm interested is because the child already has a degree of colour-blindness, mixing up his greens and reds. It sounds to me like a type of retinal defect, though I have no clue as to what type. Maybe I'm being paranoid. Thoughts?