Hi,
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?
If the swelling still persists after a few days, it would be best to consult a doctor for a clinical examination and whether a course of antibiotics is required.
Let us know if you need any other information.
Regards.
My son has had swollen lymph nodes behind his ears before. the doctor said not to worry and that it would go down on its own. and it did. However, she did say that if it did not then she would put him on an antibiotic. (He sometimes gets this when he has a sore throat, fever, cough, sinus symptoms, etc) but she was not overtly concerned about this (as i was, like you, very concerned). If your very concerned and it does not clear up within a day or two, check with your pediatrician. Good luck.