It is important to give him plenty of fluids to prevent dehydration. Orange juice diluted with water may supply vitamin C. A pint every three hours is adequate. Because food sources of vitamins are unreliable, give him a 500mg capsule of vitamin C every three hours during the illness. If his bowels become loose, reduce the dose. If the increased use of vitamin A is not made up the body's stores may remain depleted for several months.
Carrots are a good source of beta-carotene which is converted to vitamin A as needed.
Other sources of vitamin A are sweet potatoes, winter squash, red peppers, broccoli and liver. Broccoli and red pepper also supply some vitamin C. Cod liver oil supplies vitamin A and essential fats needed to modulate the immune system and reduce inflammation. If he has a liquid diarrhoea use no more than 200mg of vitamin C and don't use cod liver oil. This will supply sodium, potassium and sugar. Very ripe mashed banana is good for normalizing bowel function and supplies a healthy dose of potassium.