Eating grass can cause vomiting all by itself so you should discourage this behavior. If you are seeing yellow vomit, this usually means two things - you are seeing bile and he is vomiting on an empty stomach.
If you know that he's vomiting on an empty stomach, you can somewhat help this by never letting his stomach empty. Offer him food at night before you walk him one last time and then feed him when he wakes up. You might even want to give him a bland diet when he has flare ups. Boiled chicken and white rice is a great choice (no bones, no skin). Some vets will recommend that you go so far as offering tagamet or pepcid to help with reflux if this is an underlying cause.
This would be a good time for a fecal exam to rule out parasites as the cause of the nausea.
For immediate relief, keep him from eating any grass any let his stomach rest. Then offer some pedialyte and see if he can keep it down.
If he can't or this does not resolve quickly, give your vet a call.
Let me know if you still have questions.