I read long ago, that eating grass for some dogs is completely normal because THEY LIKE IT! Just another veggie for them, like salads are for us. I had English Setters that would KILL for a good head of lettuce! My Pogo (half Dachshund/half Poodle) has always eaten grass and doesn't throw it up. And he has taught little Sir Higgins to eat grass, and he doesn't throw it up either. SO, if your little one isn't throwing it up each and every time, then I say just let him enjoy it! But when you see your vet, do ask him about it.
IMPORTANT NOTE: Be careful that he doesn't eat grass that has fertilizers or weed killers on it! Just good ol' organic grass.......