Answer

Hi, XXXXXXX.

It's possible that your dog has ingested a toxic substance or other non-food item and this is causing him to feel the need to eat grass and throw up; it's a natural instinct in dogs to eat grass and try to purge themselves, if they ate something they shouldn't have, that's making them feel sick.

http://www.sniksnak.com/doghealth/vomiting.html

http://www.petplace.com/dogs/poisoning-in-dogs-what-you-should-know/page1.aspx

When you say he's eating his tail, do you mean he's attacking it and biting it? He may have a gotten bitten or stung by an insect, spider, snake, etc., depending upon your location, or he may have an irritation there, which is causing discomfort.

If your dog has been throwing up frequently, it would be a good idea to have him evaluated by a vet, as soon as possible, because dehydration is possible and the cause of the vomiting must be definitely diagnosed and treated so he's feeling better.

I hope all will be well.

Cher