Rescued dog vomiting after being altered.?

We just rescued a four year old golden retriever mix from the local shelter. He is quite thin and ate the food we gave him very quickly. Since yesterday, he has had some loose stool and vomiting. We tried giving him smaller amounts of food, but it didn't work. We have held off on food and he has not had anymore loose stool, but has vomited clear, foamy liquid. They vaccinated and altered him at the shelter and he appeared to be ok until brought home. I'm very worried and don't know what to do, and of course it's the weekend and the vet is closed. Please help!