A lump behind the ear
in children could be due to throat
infection, insect bite, accidental trauma to the site, ear
infection , etc.
Lymph nodes in the region are known as post auricular lymph nodes and can enlarge secondary to infection, inflammation, etc.
You could try some warm compresses at the site, warm saline gargles for his sore throat and some oral antihistamine medications for the swelling.
Does he have any other symptoms of fever, etc?