The diarrhoea could have caused dehydration which in turn might have reduced the blood volume. In turn, since the circulatory volume is less and since probably the blood pressure is also down, the blood flow to the brain is sometimes affected, especially when he gets up, a phenomenon called postural hypotension which is resulting in such lightheadedness, dizziness and blurred vision. Get his blood pressure checked immediately. If it is low, he would probably need ORS. Give him proper wholesome food and he would gradually recover. It would still be worthwhile to get a cardiological check up done.