Gypsy's idea is a good one.
A simple thing that I do is this...(ok, a lot of typing, but it surely is simple)
When he tries to take that first bite...I say "HEY!" in a quick, firm voice. If he keeps going for the grass, I then say "HEY" and give a quick pull downwards on the lead rope (it makes a noise from the snap hitting the halter ring plus applies pressure to nose and poll). If that fails, I then do the "HEY," the quick pull, and I add a lunge towards him with my body ending in a stomp on the ground near his head. That for SURE gets his head away from the grass! Like in a heard, if the alpha horse suddenly lunges towards another horse, the other horse will surely back down.
Anyways, then a few steps down the grassy road, he tries for another tasty blade of grass. So I repeat..."HEY!" If needed, I'll go again with the "HEY" and quick pull on the lead. He's probably got the idea by now: "HEY" means stop eating grass and pay attention to your handler!
I did this with my horse when I first brought him to my barn because right where we tacked up & bridled there is always a bale of hay for feeding. Well every time that I'd take his halter off to bridle, he'd dive straight for the hay. After a few days of this, he responded to me simply saying "HEY" when he veered towards the hay. Soon, I'd simply have to look directly at him and he'd behave. Now he finally just stands there. Every now and then he'll try to snag a bite, but I say "HEY" and he's back at full attention! It's also good for that occasional time when he tries to chew on the arena rail or something..."HEY!" and he stops.