Food particles or saliva is aspirated into the lungs while tube feeding. This will be unnoticed but can cause serious pneumonia.
Your Grandpa who is coughing now may has aspiration pneumonia
. Get a Chest Xay and rule out pneumonia.
You can prevent aspiration pneumonia. Make him sit upright during feeding and for 1 hour after feeding is stopped. If it is difficult to sit, keep the head of the bed elevated at least 30 degrees.