If the baby isnt sitting up and still uses the tongue to push things out the mouth id suggest you hold off on the cereal. This is my second child, the oldest is 2 going on 3 and this one is 4 months old but i regularly massage/exercise both as well as keep them upright during and atleast 30 mins after each meal. Exercise is 1 rep of leg crunch where i press gently but firmly the leg up unto the stomach or do bicyle motions with his legs 1 rep of 10. Both always poo shortly after...literally loaded diaper.
If keeping baby upright doesnt help see a doctor atleast to ensure its safe to thicken baby's formula