Phlegm after eating

My father had his upper lobe of lung and 3/4 stomach removed 2 yrs ago with cancer. Since then we have had to insert feeding tube in his stomach because he weighed 99 lbs. He is up to 126 but still cannot eat due to the phlegm he gets after he eats anything. He does not ever spit up the food he is able to eat. Which is 3 bites of soft liquid food.If anyone can help or knows what type of physician to see please let me know.