Have your parents read this thread? Even if the advice here won't convince them to take him to a vet it might confirm to them that they should not be forcefeeding.

You CAN rub the nightcrawlers on his lips to convince him to try and eat. I often do that with my frog who has had neurological issues in the past and doesn't strike at food well/has horrible aim. I apply a slight bit of pressure at the frog's lips: If she turns to one side or swipes the food away that means she's not interested and I don't push it any further. If she doesn't do that, it usually means she'll eat within a few seconds.
One thing to be careful with is just to angle the tongs in a way that the frog doesn't jab itself in the mouth going after the food.