Conveying Physical Recovery:
* Once the fever broke.
* When his temperature returned to normal.
* As his body regained its strength.
* After he had recovered from the illness.
* Upon regaining his health.
* As the sickness subsided.
Conveying Emotional Recovery:
* When the fear subsided.
* As the anxiety faded.
* Once the panic had passed.
* When he was back to his old self.
* After his spirits lifted.
More Poetic Options:
* When the fire in his blood had dwindled.
* As the heat of illness ebbed away.
* When his pulse returned to a steady beat.
* Upon regaining his vitality.
Adding More Specificity:
* After the fever had left him weak but alive.
* When his body was finally free of the illness.
* Once the delirium had cleared.
The best choice will depend on the context of your writing and the specific effect you want to achieve.