Dog has vomiting problem

I adopted a 5 year old rat terrier from teh pound. He was very thin when we got him and had a bad case of worms. Supposedly the pound had wormed him when we got him. He started vomiting from teh day we got him. initially we though that this was from teh worms, but after two trips to the pound, they gave him a pill for the worms. He has no more worms in his stool, but he still has the vomiting problem. It is not consistant with him eating, sometimes it can be right after he eats or sometimes hours after. Sometimes it is just a small spitup and others is just pours. He is still very thin and I am starting to wonder if this is teh reason that he was brought to the pound. I cannot afford to pay hundreds of dollars for treatments. I really need help here or i am going to have to take him back in. We avoided getting a puppy so that we woudl not have the carpets destroyed, but now it seems that it is happening anyway. Does anyone have any suggestions?