I need a little more information: was this baby growing well before
introduction of cereal in the diet.  When were the cereals introduced?

I a child this age who suddenly quits growing, it is very important to get
a detailed diet history and if the change in growth pattern relates in
time to introduction of cereals I would be very concerned about celiac
disease ("allergy" to gluten which causes gut damage and poor growth.  May
or may not have diarrhea associated with gut damage).
Maryelle