Observe what foods might entice your child's appetite.
It could be colors or it could be shapes or both.
Children also go through "food jag" periods.
This is the time when a child likes to eat one food all the time.
For example, a child might like to eat ketchup on everything, including an apple pie. Introduce one food item at a time.
This will help to determine if your child might have any food allergies.
Thus, you will be able to infer what foods might cause the allergies.