
My sisters Boxer is just like this, she often gets a bad belly from eating the simplest of things like a different flavoured dog treat. She is normally a typical Boxer, all play, play, play & on the go, but every now and then she will hardly get out of her bed and she refuse to eat for a couple of days and she'll be sick. She has been taken to the vets several times but they say she just has a sensetive stomach and my sister is starting to find what the triggers are now.
The white froth is probably due to her having an emty stomach, but personally, I would go back to my own vet, just to make sure.
I hope he's feeling better soon :)