my pom throw up a lot

Question
My pom does a lot of grass eating and throws up.  He does this about 3 times a week regularly.  I feed him the Nutro brand dog food, the bite size food for small dogs.  He sometimes swallow this bite size without biting it.  I also, feed him the bigger bite size food where he can bite it.  I have tried different brands of foods with the same result.  I don't think it is the food that is causing him to throw up or if it is something else.  He is 8 years old and he has been throwing up all this time.  I told my vet and they told me they can run blood test but they don't know what is causing him to throw up.  Can you help?  Thank You.  

Answer
Hi Nissar Jan,

I have a few questions for you.  What other brands of dog food have you tried?  Do you give your pom heartworm prevention, such as Heartgard or Interceptor?  Do you put topical flea prevention, such as Frontline on him?  Does he get yearly vaccinations?  Is he an indoor or outdoor dog?  

Carrie