Thank you for your query,
The bad breath
may be due to upper respiratory tract
infection may be viral or bacterial.
Give plenty of water and oral fluids to drink as it will decrease the bad breath.
Brush his teeth with a gentle bristle tooh brush and you can make him to gargle with fresh water frequently.
Continue the medication as advised by your doctor.
You can apply local benzocaine
containing gel on oral sore to decrease the pain and to increase the oral intake.
Even after 3 days of treatment if there is no decrease in fever
consult your doctor or as he advised.
Hope I have answered your query.