A: The best way to know if your son is getting enough food to meet his needs is to look at his growth. If visits to his pediatrician show he's steadily increasing in weight (he should be about triple his birth weight by the time he's 1 year old), that's a good indicator he is getting enough energy to meet his needs.
It's wise to wait until your son is between the ages of 4 and 6 months to offer cereal and other foods, since that's the age when he can sit without support and he is developmentally ready to eat solid foods. His weight should also be about double his birth weight at that time as well. If you have a family history of food allergies, follow the advice of the American College of Allergy, Asthma, and Immunology and wait until an infant is 6 months old to feed him solid foods.
As infants grow, it's perfectly normal for them to have fluctuations in their appetite and food intake. They're increasingly more interested in exploring their environment (especially when they start crawling and taking steps). Unless your son isn't meeting growth chart standards pediatricians use, rest assured he's getting enough calories to meet his needs.