If there is any possibility that he might have swallowed a non food item that would not show on x-ray that could be the issue. Cloth for example doesn't show on x-ray. Clothing, bits of a toy, plastic, splinters of wood, all can cause this.
Anything obstructing the valve leaving the stomach or the intestinal tract could cause vomiting like this.
I presume your vet has checked for pancreatitis, has x-rayed for any hard object puncturing the intestine or lodged there, has done bloodwork to check for infection, or liver, or kidney issues, or intestinal parasites etc. But has your vet suggested a barium x-ray, an endoscopic exam to view the stomach, or an ultrasound to look at soft tissue problems?
Has the vet checked for bloat?
http://www.dachshund.org/bloat_instructions.html
You might want to ask your vet about consulting with an internal medicine specialist.
You might want to ask about exploratory surgery to see what is wrong if the testing is too expensive for you.
Hope this helps you!
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