Neck swelling is due to lymph gland enlargement. This is due to source of infection in nose, sinus
More details are needed like if he gets recurrent colds, sore throat
or if he snores, mouth breaths etc to find out source of infection and what about blood test
Neck swelling has recurred because at source the infection has not cleared completely.
The bacteria may not be very sensitive to second antibiotic - Monocef
. So it is slow to subside.
So it is important to examine the nose and throat to see any sinus or tonsillar infection or adenoid
enlargement is there and treat accordingly.
So please consult an ENT surgeon.