Flossing as well as brushing is needed, and make sure he brushes his tongue, but really he needs to see his dentist to check he doesnt have any tooth decay or gum problems which are the most common causes.
If the dentist doesnt find anything, then he should see the doctor if it is persistant as it can be a symptom of various health problems.
This is a useful website to see if you can work out causes and treatment.
Sounds like he has good oral hygene so perhaps there is an underlying problem there.
There is a product specifically for halitosis called Retardex that you can get from Boots or big supermarkets. It's a range including a mouthwash, spray and toothpase. I recommend the mouthwash.
I used to suffer from bad breath; I was always conscious of it and made sure I didn't breath on anyone! I then sought advice from a hygienist, who got me using little brushes between the teeth. It's amazing how much rotting food emerges, even after a thorough brushing with a toothbrush! Mouth washes etc are ok to treat the symptoms but you need to get to the cause. Clearing out the spaces between the teeth will also make the gums and teeth healthier.
There are lots of mouthwashes and so forth on the market, but these can erode the enamel on your teeth with constant use. The first thing to do is definitely visit your dentist. Tups and Kira are right. It's most often the plaque that gets stuck between the teeth which cause bad breath. You also need to use an electric toothbrush on the back of the teeth, and to use a pick after every meal. http://www.teethnthings.com/catalog/SoftPicks. jpg
You can buy these in large stores such as Boots, as well as tongue scrapers.
Yuck - teram!!!!!
You're right about diet - smoking and drinking as well - but my poor gran had the start of gingivitis, and was advised by her dentist TO brush very frequently and well. It's more important to get the plaque removed from between each tooth though.
persistent bad breath can be indicative of a gastro-intestinal disorder or throat infection. If your son practices good oral hygeine as appears to be the case, perhaps get these two things checked out too.