Rice Cereal in formula

When is a good time to introduce rice cereal? My son is constantly screaming for food. He could have just had a bottle (4oz) and he'll eat another 2oz and still scream for more. I've tried putting his paci in his mouth but he spits it out and cries some more. I change is diaper, then rock him some, and try a whole bunch of other things but he's just so hungry.