My 2 year old pug has had vomiting issues that we've tried to tackle from a variety of angles. At his worst he was drinking a bunch of water and throwing up immediately afterwards. We were constantly taking him to the vet, and on the last visit she gave him a pepcid shot, an anti-nausea shot, and a water/electrolite pack under his skin that would keep him hydrated as we had him fast for an evening. For the next week we gave him a vet prescribed bland-diet of rice and cottage cheese. That and hypoallergenic food from then on seems to do the trick.
Maybe he has developed a food allergy? Also, he might be eating too quickly. Is there bile in the throw-up, or is it just the food? Just the food suggests he might be eating too much too fast. If he is immediately throwing water back up, cut the water and food for a bit to give his stomach time to settle.
The best thing you can do for him is get him to the vet so they can test him for stuff, though.
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