Thank you for your query on Healthcare Magic.
Your child seems having an upper respiratory tract
infection could be due to a bacteria or virus.
in prescribed dose.
Ask him to gargle with salt water.
Give plenty of oral fluids to drink.
If the fever is persisting, antibiotic may be required.
The dark line is looking like a superficial vein
in the skin on chest.
Consult the doctor again if fever is not subsiding beyond 48 hours of infection.
Hope I have answered your query.