Talk to Pediatrician on Baby Refusing To Eat At All

It has been over a month my son is not eating anything. He is also losing weight so fast.i can tell from his lose clothes. He is already an underweight baby from birth. Birth weight 2.19kgs. Before this hunger strike his weight was 8.7kgs. No matter what I offer him he is not eating. Except for slanty or lays. I always gave him home made food. Never even gave him cerelac fearing he will refuse homemade food. But now I am so worried and exhausted. Please tell me what should I do???