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Swelling behind ear with redness will be due to inflammation. Possibilities
are either post auricular abscess, post auricular lymphadenitis (inflammation
of lymph node) or mastoiditis (inflammation of bone behind the ear).
With redness and swelling usually it should be associated with pain and fever.
I advise you to get him examined by an ENT specialist as physical examination is required to make a definite diagnosis.
Antibiotic should be started including Amoxicillin 250 mg three times daily for 5 days after a meal for all of the above mentioned conditions.
Adequate anti-inflammatory and analgesic including Ibuprofen 200 mg or paracetamol 500 mg can be given two times daily after a meal for 5 days.
If it is an abscess, then incision and drainage may be required.
Hope this helps. Feel free to ask if you have any further queries.

Thank you.