You can get supplements for children which will help them , your local drug store should have a range to chose from check the age on the bottle .At 3 year old children are often'picky' , try cereal for breakfast, yoghort, fruit, let him eat himself, put the food down and let him get on with it.Pasta they do kids shape pasta , noodles,beans, eggs,chicken, soups are all good for them, if you put eating/diet for a 3 year old into the search engine there will be a list of good foods to give him .Don't fill a 3 year old up on milk as that often takes their appetites away.