There is a condition called Cyclical neutropenia where approx every 30 days, certain white blood cells reduce in number. Without normal white blood cells, there is a risk of viral or bacterial infection.
This website has more info:
http://www.healthatoz.com/healthatoz/Atoz/ency/neutropenia.jsp
Other cyclical causes of fever are Brucellosis and Malaria, although its unlikely that he has had exposure to these conditions.
When he is in an episode, it may be worth asking the doctor to send blood samples off for viral and bacterial culture to see if anything can be identified. Other causes of recurrent fevers are Ebstein Barr virus.
Aside from this, ensure he is eating plenty of fruit and veg to ensure he is up to daily recommendations for vitamins and minerals.
Regards
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