Hi mjs pfi,
If there were a tooth problem the dog would probably not be eating. But check for discolored teeth or gums. There may be something going on there. Dogs will foam if they eat to fast. Try slowing down the eating habit by putting an unopened can of vegetables in the middle of the dish so the dog has to eat around it. If this works then there are bowls with cylinders already in them for just this reason. Also add some water to the dry food. If the food swells when water is added then wait for the swelling to occur before giving it to the dog.
I'm a bit confused, are you hand feeding the dog one kibble at a time? Will he not eat a bowl placed in front of him?