Dermatology Questions? Ask an Online Dermatologist.
This is called melanonchyia. The vast majority of these are benign, most due to a mold under the nail plate. Nonetheless, I would recommend that he consult a dermatologist before going back to school.
The dermatologist should be able to tell whether this is a benign pigmented streak or early melanoma. A special light called a dermatoscope can help him/her do so.
There are signs to look for. If the edges of the pigment are sharp; if the color is even throughout the streak, if the color is not too dark....these are all signs of benign melanonchia. Also, if there is no pigment spilling up on the skin ( Hutchinson's sign)..that is good. If the streak does not expand...that is good.
It would be wise to photograph it. This way, photographs can be compared.