DS is 16 months and caught a horrible cold with a chesty cough. He has not eaten more than a handful of raisins or cheerios for over four days now. He's drinking his milk and water ok, but won't eat anything else, even the 'treat' type stuff like the Goodies fruit bars, fruit loaf, biscuits etc. He hasn't got a temp but I think he may be teething too. How long before I should start to really worry?