Hi nspeth :)
Sounds like your child has a gluten intolerance by the symptoms you mentioned. An intolerance to the gluten, found in wheat, barley, oats, and rye.
About 1 in every 250 people have it. Some forms are mild and some a little more severe. depending on their reaction to gluten foods.
I suggest you give feed your child some buckwheat.
It is a matter of eliminating the offending foods, some do not have a problem with minor glutin foods.
Grains contain gluten that is common in some other protein foods such as walnuts etc.
Forbidden List - Foods and Ingredients Not Safe for a Gluten-Free Diet
http://www.celiac.com/st_prod.html?p_prodid=185&p_catid=12&sid=91hH9H1UtCXwE9...
Abyssinian Hard (Wheat triticum durum)
Alcohol (Spirits - Specific Types)
Amp-Isostearoyl Hydrolyzed Wheat Protein
Barley Grass (can contain seeds)
Barley Hordeum vulgare
Barley Malt
Beer
Bleached Flour
Blue Cheese (made with bread)
Bran
Bread Flour
Brewer's Yeast
Brown Flour
Bulgur (Bulgar Wheat/Nuts)
Bulgur Wheat
Cereal Binding
Chilton
Club Wheat (Triticum aestivum subspecies compactum)
Common Wheat (Triticum aestivum)
Couscous
Dextrimaltose
Disodium Wheatgermamido Peg-2 Sulfosuccinate
Durum wheat (Triticum durum)
Edible Starch
Einkorn (Triticum monococcum)
Emmer (Triticum dicoccon)
Farina
Farina Graham
Filler
Flour (normally this is wheat)
Fu (dried wheat gluten)
Germ
Graham Flour
Granary Flour
Groats (barley, wheat)
Hard Wheat
Hydrolyzed Wheat Gluten
Hydrolyzed Wheat Protein
Hydrolyzed Wheat Protein Pg-Propyl Silanetriol
Hydrolyzed Wheat Starch
Hydroxypropyltrimonium Hydrolyzed Wheat Protein
Kamut (Pasta wheat)
Malt
Malt Extract
Malt Syrup
Malt Flavoring
Malt Vinegar
Macha Wheat (Triticum aestivum)
Matzo Semolina
Mir
Oriental Wheat (Triticum turanicum)
Pasta
Pearl Barley
Persian Wheat (Triticum carthlicum)
Poulard Wheat (Triticum turgidum)
Polish Wheat (Triticum polonicum)
Rice Malt (if barley or Koji are used)
Rye
Seitan
Semolina
Semolina Triticum
Shot Wheat (Triticum aestivum)
Small Spelt
Spirits (Specific Types)
Spelt (Triticum spelta)
Sprouted Wheat or Barley
Stearyldimoniumhydroxypropyl Hydrolyzed Wheat Protein
Strong Flour
Suet in Packets
Tabbouleh
Teriyaki Sauce
Textured Vegetable Protein - TVP
Timopheevi Wheat (Triticum timopheevii)
Triticale X triticosecale
Triticum Vulgare (Wheat) Flour Lipids
Triticum Vulgare (Wheat) Germ Extract
Triticum Vulgare (Wheat) Germ Oil
Udon (wheat noodles)
Unbleached Flour
Vavilovi Wheat (Triticum aestivum)
Vegetable Starch
Wheat, Abyssinian Hard triticum durum
Wheat Amino Acids
Wheat Bran Extract
Wheat, Bulgur
Wheat Durum Triticum
Wheat Germ Extract
Wheat Germ Glycerides
Wheat Germ Oil
Wheat Germamidopropyldimonium Hydroxypropyl Hydrolyzed Wheat Protein
Wheat Grass (can contain seeds)
Wheat Nuts
Wheat Protein
Wheat Triticum aestivum
Wheat Triticum Monococcum
Wheat (Triticum Vulgare) Bran Extract
Whole-Meal Flour
Wild Einkorn (Triticum boeotictim)
Wild Emmer (Triticum dicoccoides)