I doubt that you have lymphoma. Typically that is not a node that you would have swell up due to lymphoma. Those nodes are in the neck, underarms, groin, chest and abdomen. I am sure that the problem is due to the rash.
A lump behind the ear 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.
This could be mastoiditis, cervical lymphadenopathy or parotitis.
Have you seen an ENT specialist or your doctor for your symptoms? Do you have any associated symptoms of pain, discharge, fever, etc?
'Mastoiditis is an infection of the mastoid process, the portion of the temporal bone of the skull that is behind the ear. It is usually caused by untreated acute otitis media (middle ear infection) .'
You could read more about the condition at the following links -
Let us know about how you are doing and if you have any other doubts.
Post us about what your doctor advises.