Has your vet done any blood work to rule out things like kidney failure or liver disease? Clear or white foamy vomit is a common symptom of Feline Chronic Renal Failure (CRF.) If your cat likes canned food, you can mix water into it, to make kind of a slurry. Also, most cats love baby food meats (make sure it contains NO onion.) You can also add water to that to help increase his fluids. You can also try giving your cat 1/4 of a 10 mg Pepcid tablet (famotadine) to see if that will control the nausea. Do not use Pepcid Complete, because this contains other ingredients that your cat doesn't need.
On Tuesday, talk to your vet about anti-nausea drugs, such as the Pepcid, or Reglan, or even Zofran (Zofran is quite expensive, so it probably wouldn't be the first choice.) My Angel kitty, Spooky, was a chronic vomiter, due to chronic renal failure. I gave her 1cc of Reglan syrup, twice a day, for the last 4 years of her life. It controlled her vomiting very well. My parent's Angel kitty, Samson, was also on Reglan syrup for nausea/vomiting caused by CRF. He successfully took it for at least 3 years, before he finally succumbed to other complications caused by his renal failure.
Also talk to your vet about learning how to do sub-Q fluids. I've been doing them for over 6 years for my CRF girl, Cleo, and I did them for about 4 years for my parent's Samson. They are very easy to do, and they are a God-send for preventing dehydration that can be caused by CRF, chronic diarrhea, or chronic vomiting.
Good luck! Please let us know what you find out on Tuesday. I will keep you and your kitty in my prayers.