I've read that a side effect of the keto diet is bad breath. Subtly start a conversation about keto. Maybe he's trying it. OTOH if he's a fat f%@#, that probably isn't the reason. Maybe his breath smells like Doritos.
What a gross thread. Maybe an anonymous note is the best way to put the person on notice. A lot of people are too thick to get the breath mint hint.
This can be a mouth problem or a stomach problem.
As people get older, they get bigger gaps between their teeth, and just ONE little chunk of decomposing chicken or fruit or a bit of popcorn is all it takes. You sleep with that overnight, the little bacteria do their thing and you wake up in the morning and your breath is ... well you shouldn't come within 500 feet of another person. Even if you floss several times a day, all it takes is to miss something for a few hours and your breath is not fit for public consumption.
People can be unaware of this -- the dentist will point it out, but a lot of people don't do regular dental visits.
The other cause is reflux, which tends to happen when people get to be 20-40 pounds overweight.
Reflux means your stomach contents are getting pushed up past the pyloric valve into your esophagus. In many people this causes heartburn so they know something's wrong. But some people don't feel pain. Or they take long-term antacids so they don't get a burning sensation -- but the reflux is still happening.
Anyway, those stomach contents will sit there in your esophagus and the bacteria do their thing and it smells awful.
The reason it smells like s--- is because these are the same type of bacteria that live in your gut and help decompose your food. They produce methane and when they die, they BECOME poop.
Diet is part of this. A very sugary or starchy diet or drinking a lot of soda will increase the volume of bacteria in your mouth and esophagus and make you smell worse.
The Keto diet can make your breath a little funky. It can definitely make your pee smell weird a little like eating asparagus (it's the smell of ketone byproducts). But that isn't the breath that is going to cause people to stampede out of meetings to get away from you. Breath THAT had is from decomposing food in your mouth or esophagus.