Bad breath. What is this actually?

My son who's 5 years 10 months old . he has bad breath even after brushing for 2 times a day since some times. I thought it's because of decayed teeths. But since last 2 months he was having cold cough.. and also got fever 2 weeks back so I took him to ENT doctor they gave some medication which I have uploaded.. after taking that medicines for 3 days . His mouth started smell good. No bad breath at all even after waking up in the morning he was not having bad breath. But from yesterday it again started smell bad again. Am frustrated what is this. What should I do. ? And also I have noticed when he get cold his mouth doesn't smell bad .after he get better his mouth smells again. Please help .what is this actually?