Why doesnt my brother eat anything?

ok well i have a baby brother thats 17 months old and he wont eat. we went to the doctor to tell him and he said to not feed him milk so we didnt. we waited eight hours with no milk he was hungry by crying so we gave him food and he just throws it. so we had to give him milk because we cant starve him to death! and when he was around 11 months old he ate alot but not since 6 months he wont eat maybe one tiny bowl of soup per week. otherwise he just eats like baby cheetos but still not much. but to food he never wants any. if we feed him in his mouth he gets nausea and throws up. we asked the pediatrics to give us vitamins to replace his nutrition but they keep telling us to not feed him milk but it doesnt work he could go a day without eating until he gets his milk! please help we are worried that without the 1000 calories he needs a day he might get like blood cancer can he for no vitamins? and why wont he eat he only loves his milk help!!!!!!