Your son is likely to be having Lymph
nodes on the back of the skull.These Lymph nodes increase in size when there is infection of the scalp
,ears,neck and throat.In your son recurrent Upper respiratory tract
infection must have been the cause of Lymph node swelling.
If there is no fever
now and he is fine and active,these lymph nodes would disappear in few months.However if it increase more than 1.5 cm and are painful then get him examined by the doctor.