Cats tend to eat grass to aid them in getting rid of hairballs. Is there evidence of hairballs when he throws up the grass? Do you give him anything to help prevent them? Maybe the hairball paste or certain treats? Have you thought of trying one of these?
My two eat a lot of grass too, but they do tend to keep it down - unless the pieces are too long and not properly chewed.
It may be worth trying the paste to see if this helps. It might improve his appetite once the hair has gone. (I'm no expert, mind you)
Also, does he drink enough? He might be eating the grass for the moisture. Also, you might want to try giving him some wet food, maybe only twice a day, but i've heard from a lot of people here - who dry feed - that they do this.
Hope this helps a little bit, but I'm sure others will chip in soon!