Thanks for writing in.
The history you are giving is only part of symptoms. I would like to know if he has fever , loose motions, or other specific symptoms like stomach pain.
Unable to eat or drink would have lead to dehydration
and can cause greying of corners in mouth. Any lesion in mouth like small ulcers can cause difficulty in eating. Even pharyngitis
can cause same symptoms. So we should find out what is the underlying pathology for not eating or drinking.
Sometimes worm infestation or viral fever
can cause similar symptoms. I would recommend you to consult pediatrician
and rule out causes for illness.
I hope I have clarified your doubt. In case required, please revert back for further clarification.