Usual lump on the neck is due to lymph nodes.
Do you see any pustules or any skin infection
over his face, scalp etc? Or does he have lice infestation
on his scalp? These may cause superficial nodes to enlarge and will go off once the cause is treated.
in the neck can get enlarged due to throat infection
, tooth caries etc. These too go off on treating the infections.
But usually lymph nodes will be mobile and firm to touch.
If you are sure that it is hard to touch, have a word with your pediatrician
Examination by a doctor will take away your doubts and obsessions.
Hope you find my answer helpful.