Thank you for your query.
It would have been better had you mentioned the three possibilities given by the doctor so as to give you a guidance. Without having a look at the child it is very difficult to hazard a guess. However by your description it appears to be a pre-auricular lymph
node which may have been enlarged due to a focus of infection in the anterior scalp
It is very difficult to say how long will it take to subside without having a look at the lesion. Apart from the antibiotic you can add some anti-inflammatory medicine to help it subside.
I hope I have answered your query to your satisfaction.
Wishing your son an early recovery.
Rajiv K Khandelwal