I'd not panic unless you see repeated vomiting and / or diarrhoea. He's probably feeling a bit 'off-colour'. Dogs eat grass to help sort of uncomplicated tummy upsets, and sometimes they will vomit, sometimes they don't. Mine will often tug at the new shoots after Winter and rarely do they vomit - we just call it having their salad.
If you have a rectal thermometer and if he does seem less active then usual, it might be worth taking his temperature, and of course, if you feel you need to have your vet take a look at him, if only to set your mind at rest, do, but for now, I'd just perhaps cut back on his food for 24 hours, or even withhold, bringing him back on boiled rice and chicken + some natural yogurt. Make sure he does have access to fresh water, and is drinking.
ps Dogs tend to vomit far more readily than we do.