Sneezing is caused by an irritation in the nasal passages.
If the sneezing occurs while swallowing, a small amount of cereal might be "leaking" up into the back of the nose, causing irritation. Make sure the cereal isn't super thick, and slowly thicken over time. The symptoms should improve. If not, then you should be seeing the pediatrician/family doc for the baby's 6 month exam soon.