Eating grass is pretty normal. There is no real evidence as to why dogs do it. He could be eating grass because he has an upset stomach or he might just like the taste. My dogs love rose bushes and eat the heck out of the roses near the bottom. You can google "dogs eating grass" and read all the differant sites, but I really don't think you have anything to worry about.
Usually my dog will eat grass when her stomach is sick. It makes her throw up and then she feels better. When she does that I usually make her boiled chicken and rice for a day or to and that helps calm her stomach down.
My goal in life is to be as good of a person my dog already thinks I am.
My dog eats grass almost every day (that he can--there's no grass here in MN at the moment). If he goes at it like a lawnmower, I believe he wants to make himself puke (for one reason or another). If he just eats a few blades here and there, I think he's just eating it for the taste, 'cause he doesn't puke. I don't think it's harmful, and I think it may fulfill more than one purpose.