First of all, feed your son when he's hungry. This is a very common concern of first time moms that they are overfeeding. If your son is hungry, he's hungry.

Follow his cues, he won't over eat. Babies this young only eat when they are hungry. He might be going through a growth spurt also. Second, a 3 week old should not be getting cereal. That is WAY too young. The only time a baby under 3 or 4 MONTHS should get cereal is if a dr has recommened it for reflux. Cereal should not be given to a baby that young to make him sleep longer either. Babies eat so often because they need those nutrients to grow stong and healthy. In my opionion, it is VERY irresponsible to give a 3 week old baby cereal AND to let them sleep for 7 hours straight.