There is dental problem with swelling on the left side of cheek causing trouble. It will require clinical examination& a small xray of the suspected teeth in question, also in the meantime you can ask him to rinse with lukewarm salt water after every meals to maintain oral hygiene & apply ice pack on the affected cheek from outer surface I. E on the left cheek .
Next Steps
visit dentist for further clinical examination
Health Tips
Don't worry about it. it's has solution.