He is eating grass because he is sick. Dogs only eat grass to flush out their system to make them better. A dog usually eats the grass then vomits it up, so he can get rid of everything in his stomach. He may have worms, or he ate something bad. Other signs of worms are: 1: Bloated stomach.
2: Pus in the eye.
3: Eating all the time and always hungry or his hunger decreases dramatically.
4: Scooting his bottom across the solid surface.
5: Worms or white stuff in his poo, or blood and grass.
6: Maybe eating grass
If he ate something bad, he will be flat, looking sick, goes off his food, eating grass all the time and vomiting and possibly pale..
I suggest you buy a worm tablet for the dog and if the eating grass doesn't stop, then he ate something bad, but if he ate something bad, the symptoms should clear up within a day or two. If it doesn't stop, then you should take him to the vet.
I hope I helped.
Good Luck.