Hi,Dear.Thanks for the query to HCM virtual Clinic.
I am Dr.Savaskar attending on your query.I studied your query and understood your health concerns about your Sister.
Cause of lump behind ear-
Your son seems to have post-aural Sebaceous /post-aural
Dermoid Cyst,as it has not reduced after antibiotics.
So he should consult ER Surgeon,who would fix the diagnosis and would treat the lump accordingly.
This will help you to plan treatment with your doctor.
Hope this would resolve your query.
Write good review and hit thanks after reading this reply.
Welcome for any further query to HCM and to ME.
Wishing you early recovery.
Dr.Savaksar M.N.
M.S.Genl-CVTS,
Senior Surgical Consultant.