My dog is believed to be half pitbull and half someything else.he hasn't eaten in about 3-4 weeks.When I feed him he only eats a little bit and spits it back out whens he's done.He's been eating grass every once in a while.he throws up every two days and its different colors eachtime.His ribs are showing and he's getting really skinny!What should I do?

Get him to a vet. When a dog eats grass it is a sign of stomach upset. They do this specifically to throw up.

There is something wrong, could be worms or an intestinal blockage.

3-4 Weeks!!

Vet. Immediately. Go Now.

When it eats grass and vomits...
It could help in cleaning out its system due to the fiber the grass has..
But not eating at all is not good...
A vet would be the best way to go...
ASAP

Hi,
You haven't taken him to a Vet yet??
He could have an infection, or could be many things.
PLEASE get him to a Vet now; before he starves himself to death.
Best of luck.