i think is because of spit when you sneeze. sometimes when you sneeze you accidently spray some out and that goes into the air. after few seconds you'll be able to smell something smelly. I've experienced that when i sneezed in the car and lots of droplets from my sneeze went everywhere inside the car. then everyone started to roll the windorws down cos it smells so bad
It could easily be a chronic sinus infection. I would tell him to see his doctor so that he or she can examine them or refer him to an ENT (ear, nose and throat doc) if need be for further testing and treatment. Perhaps some xrays of his sinuses might shed some light. If it turns out to be an infection, antibiotics are usually prescribed. I personally have chronic sinus trouble and am on antibiotics frequently for my problems along with sinus rinses (they help clear the gunk tremendously, if his doc clears this, this would be a good option for him as well)
Good luck,
Sunny