Foul smell can be a result of cavities.Their might be some food lodgemnet in cavities which can cause foul smell. If he is not felling pain then cavities are not that much deep. So get them treated as soon as possible.
Next Steps
(-) Visit a doctor or video consult
(-) Brush twice a day
(-) Rigorously rinse mouth with normal water whenever eat something
(-) Drink a lots of water
Health Tips
Best way is to visit a dentist or video consult
Stay safe n healthy