--do you mean he has EBV (Epstein Barrr virus)? The top line of the post says ABV, and I don't know any acronym like that, but then later you mention EBV . . .

A number of things about this. Epstein Barrr Virus, also known as Human Herpes Virus 4, is so ubiquitous that immunologists have joked EBV should stand for "everybody's virus". More than 95% of people in the Western world have been exposed to it by age 35--usually, their first exposure results in mononucleosis or a similar type illness. (These initial exposures tend to be much less severe in yonger children, so if you've worried about the 4-year-old getting it, it might actually be a better idea at taht age than later, similar to getting chicken pox, which is anohter herpes family virus>)

Like all human herpes viruses, after the initial exposure, the cirus becomes immortalized in the body--in the case of HHV4/EBV, in the B-cells of the nasopharynx--so that one "carries" it for life (and there are antibody tests that can indicate this). Usually it is held in check by the immune system, but under conditions of stress or immunocompromise, the virus can "reactivate" and cause a number of health problems. It has been implicated in two rare cancers--nasopharyngeal carcinoma and Burkitt's lymphoma--and in a number of autoimmune molecular mimicry syndromes, including peripheral neuropathy. It has also been implicated, probably through a similar mechanism, in the development of multiple sclerosis.

For a while in the 80's people thought of EBV as being behind chronic fatigue syndrome, but recent epidemological research seems to indicate people with chronic fatigue are not any more likely to have EBV reactivations than those without chronic fatigue symptoms, and, moreover, many people without chronic fatigue do show reactivation profiles. The association seems to be stronger for human herpes virus 6 and chronic fatigue than for EBV.

The Wikipedia article, though technical, is actually a pretty good primer:

http://en.wikipedia.org/wiki/Epstein-Barr

There's also some good info, including a summary of the testing involved to determine if one has a new, dormant, or reactivated infection, in the CDC material here:

http://www.cdc.gov/ncidod/diseases/ebv.htm

What, precisely, do you think EBV may be causing in him? The constellation of symptoms there does not seem to be very characteristic, unless there was some sort of neurological damage with his initial infection.

(BTW, welcome to Neurotalk.)