Never use over the counter medication
particularly for pediatric age group. Bad smell from mouth and infected gum with crack may be due to infection in tooth (tooth abscess
I would suggest you to consult a dentist
for proper clinical evaluation and to assess the severity.
Upgradation of antibiotic may be needed if there is no improvement or any progress in symptoms,as he has already received 7 days of antibiotic. Till then continue the same antibiotic and do warm saline water rinses( that will relieve pain and allow cleaning).