My two-year-old keeps food in his mouth for long periods of time and spits it out when we aren't looking. Recommendations?

Here’s an example. On our way to my parents’ house, we stopped to get something to eat. When we were done, we didn’t notice that our son hasn’t swallowed his last bite of food. We get in the car and drive for another hour. He is still has the food in his mouth when we get to grandma’s house. He spits it out on grandma’s kitchen floor while we are busy talking to her.

When I say a long time, I mean he can have food in his mouth for hours at a time, then he’ll randomly spit it out on the ground wherever he is at the time.

We have to check his mouth for food after every meal!