There is some kind of a little 'mite' that can get under the skin and cause this type of problem. The mite can usually be found on farms, or where animals are raised - even the Zoo, if the children are allowed to touch the animals and the animals are 'dirty'. I do not remember what the mite is called, but your dermatologist may know. If the child cut his nails too short and broke the seal between the nail plate and the nail bed, the mite (same as fungal or bacterial spores) can enter the site and set up housekeeping. If you do not believe it to be an allergic reaction, you might try asking about a 'mite' invasion. They seem to stick to one local site and not move from finger to finger.