Maybe it's because he has a dry mouth. Dry mouth bad breath is an actual medical syndrome. It's far more common than you might think.
Dry mouth bad breath might be for simple reasons (e.g. sleeping with your mouth open all night, or drinking too many diuretics such as coffee), but it could be an indicator of serious medical conditions too.
One possible reason for your dry mouth bad breath might be due to a malfunctioning adenoid. And adenoid, according to the Britannica Encyclopedia is a mass of lymphoid tissue, similar to the tonsils, on the back wall of the nasal pharynx. If the adenoids become infected in childhood, their inflammation can obstruct nasal breathing and sinus drainage (promoting sinusitis) and block the eustachian tubes that connect to the middle ear (setting the stage for otitis). Surgical removal of enlarged or infected adenoids may be recommended.
Definitions of adenoid here.
It's good to remember that our sense of smell isn't completely subjective and accurate. So if you're worried about dry mouth bad breath or bad breath in general it's good to check in with your dentist. Visiting a dentist twice a year is recommended. Flossing is also an important part of your teeth cleaning routine, not just brushing your teeth.
PS Brushing your teeth might have other benefits too. Read this Daily Mail article which states latest studies which show cleaning your teeth twice a day might keep a heart attack at bay. More than 11,000 people's data were used in this latest study from the UK. Researchers have found that poor dental hygiene can lead to bleeding gums which lets in up to 700 (SEVEN HUNDRED, READ THAT AGAIN!) different types of bacteria enter the bloodstream which increases the risk of heart attack whether or not you're healthy, old, in the normal risk categories or not.
Read here for a proven way to fix bad breath.
Here's to your dental and overall general health.