Is he indoors? Outdoors? Both? How old is the cat? Any history of ANY health issues?
Whether it is diarrhea or vomit, because it is too frequent, kitty must be taken to the vet ASAP. Dehydration, among other issues are affecting your cat's health right now. What are you feeding him? Have you changed his diet lately?
Have you actually observed him vomitting or are you just assuming it is vomit? It sounds more like diarrhea to me and the white grains are worms.