Your son sounds dehydrated, so keep away completely from sugar and salt. If you think there may be an allergy problem, take him off all dairy and soya and feed him stewed fruit, steamed brightly-coloured vegetables (brocolli, beetroot for example), rice and other carbohydrates including avocadoes, bananas etc (but no sugars), eggs, if he'll have them, and fish. Do all you can to keep away from additives and ready meals, and buy your veggies from a good supplier - preferably an organic supplier or reputable high quality supermarket. Buy the best. It will work and is worth the effort and expense. You will see the results in a week. The food is delicious. If you can let him help you prepare the food, he will be more interested in it. He can wash it, even cut it up a little if shown how, prod it with a sharp knife to see if it is cooked enough, mash bananas, avocadoes etc. No drinks except water - absolutely no sweetened drinks. A straw in a little glass of water makes drinking water more attractive for children who are used to sweet drinks. You can slightly flavour the food, but keep the salt and sugar quantities tiny.