first of all i would like to know whether before the presence of spots he had a lot of exposure to sun? whether the spots are increasing in size or darkening in colour? whether they are appearing on any other areas? does he have any small lumps over his body?does your family has any history of neurofibromatosis?
sometimes the brown spots may occur due to sunburn
and any more exposure should be avoided. they will heal on their own.It might be freckles which sometimes occur around puberty
. In case you notice any darkening of colour or lupms on skin please consult your doctor.
You are free to ask any other query you have.