Eating grass is a fairly regular thing around here and as has been said, it seems to be a mechanism to help them throw up. When mine throw up it is usually a lot of bile (and the grass), so I'm thinking that the stomach bile is what is bothering them.

I'm more concerned that he hasn't eaten his regular food. Sometimes grass can irritate the throat too. If things don't change, you might want to make a trip to the vet.