No specific foods. There is no age of the child listed, but in general, no foods are harmful or helpful for children with asthma. Obviously, avoid any foods your child has been shown to be specifically allergic to by his physician. Some general nutritional factors have been shown to be important in asthma, particularly obesity. Vitamin d deficiency may also contribute to worsening asthma control.
Omega-3 fatty acids. No foods need to be avoided if your child doesn't have food allergies. There is some evidence that increasing the amount of omega 3 fatty acids - found in fish, flaxseed oil & borage oil - and vitamin d - found in sunshine and added as a supplement to cow's milk - in the diet may have beneficial effects for children with asthma as well as eczema and allergic rhinitis.
Regular diet. Regular diet - just like any other child.