Drat.

Your vet has given you something that doesn't work very well for nausea in cats. Reglan, aka metoclopramide, increases speed of gastric emptying and can help with gut motility problems but cats have very few of the receptors it targets for nausea control. (Note: Reglan can be of help if the cat is nauseated due to constipation but only because it helps to resolve the constipation; the nausea subsides because the cat's no longer backed up.)
I had the very same problem as this when Saoirse had her first pancreatitis flare. I had to request specific medication from our vet.
The medications you need are either generic ondansetron (the branded version, Zofran, is much more expensive) or Cerenia (aka maropitant). I suggest you contact your vet ASAP, say that the Reglan is doing little or nothing to help with Wenz's nausea and that he is still not eating anywhere near enough food. You could then request a prescription for ondansetron or Cerenia. While response to the different meds varies from cat to cat, ondansetron may be the better one to try first. (It worked better for Saoirse and, from my reading of other cases here, it often works better than Cerenia for a lot of cats recovering from DKA or going through a pancreatitis flare.)
If your vet is not familiar with veterinary use of ondansetron, here is a reliable reference to cite (and perhaps email a copy of the document to your vet if needed?):
IDEXX Feline Pancreatitis Guidelines
The above document from the highly respected IDEXX Laboratories discusses all effective treatments for feline nausea. From the document:
[Emphasis mine]
Generic ondansetron is a human medication (frequently used to treat nausea in chemotherapy patients). If your vet doesn't carry ondansetron it should be possible to get a written veterinary prescription and fill it at a regular pharmacy (ring round to check which local pharmacies stock it, or how quickly they can order it in for you). The dose my vet prescribed for Saoirse (4.5kg/10lb cat) is: 1-2mg ondansetron every 8-12 hours to a maximum of 4mg per day.
It's vital to get a post-DKA kitty eating properly as fast as possible both because it needs the calories and because it needs enough insulin in order to ensure a solid recovery and prevent relapses. Obviously if a kitty can't get enough food down it doesn't get enough calories and also it greatly complicates insulin treatment.
I hope your vet will be able to help. (I had to be a bit persistent in order to get an ondansetron Rx for Saoirse, and other members have reported similar experiences, but it's so worth it to get a treatment which really helps the kitty to eat again!)
Be sure to keep us updated with developments.
Mogs
.