Son stopped eating

My severely disabled son (24 years old, had meningitis when 4 weeks old) about 2 years ago stopped eating. He had a big appetite and suddenly would not open his mouth and it was difficult to feed him. He has no speech,vision or mobility and is wholly dependent for all his care needs and was unable to express the problem he was experiencing.Was admitted to hospital and underwent endoscopy and nothing found. He had MRI scan everything was normal. Discharged home we persevered and the only food he could take to was very limited( weetabix for breakfast, weight watcher's rice pudding for lunch and quorn cottage pie/ creamed potatoes with beans) in nutritional value. With any other foods he is in pain ( possibly stomach cramps). May be IBS? But such a limited diet, GP thinks may be not.

On and off he continued limited eating only the above foods, now suddenly again will not open his mouth to eat.Had his teeth checked out - fine. We really do not know what is wrong. He is in pain soon as the food hits the stomach. He clinches his teeth.What could the problem be? GP is referring him for further investigation?