Hi, Thanks for writing in. Mastoiditis is infection in the mastoid air cells (temporal bone) and collection of fluid within due to chronic middle ear infection (CSOM). It is important to know if he ever had any symptoms of ear infection like pain, deafness or ear discharge recently. This need to be discussed with an ENT surgeon. The small swelling behind the ear needs to be evaluated clinically. It may also be an acute infection and needs urgent medical attention. If the infection enters the brain then it can get serious.