Welcome to HCM,
It seems that your son is having bacterial infection
on his scalp
This might be associated with dandruff
, sebohrric dermatitis.
The bump you noticed might be enlarged lymph nodes due to infection.
and get him examined.
After diagnosis he will go for treatment accordingly.
Wash his head with antiseptic lotion, keephair clean and apply antibiotic cream.
Ok and bye.