My Child refuses to eat!!

My 4 year old refuses to eat! This has been going on for a couple months now. He priorly would eat his favorite foods (mac n cheese, pizza, hot dogs, chips) now he simply refuses to eat everything! We sometimes have to get him to eat by physically feeding it to him like a baby he likes to stare into space rub the table or rill his eyes at us during every meal! He weighs 27lbs and you can feel his bones when holding him. We have even tried carnation instant breakfast and nothing is working. We are about to go insane with frustration! Could this be a medical issue??