The long sickness has turned the boy flimsy.

Solution (By Examveda Team)

Flimsy : insubstantial and easily damaged.
Healthy : in a good physical or mental condition; in good health.
Strong : able to withstand force, pressure, or wear.
Agile : able to move quickly and easily.
Wholesome : conducive to or suggestive of good health and physical well-being.

Antonym of Flimsy is Strong