I am not a vet  but IMO Yes it sounds like a dental problem  but it is good
that you are taking him to a new vet

Right now  fix him some chicken broth ( no onions)  rice yogurt or pudding
baby food (cat likeable flavor no onions of any type).  At the pet store
they make just gravy you can try to give him some

Check the inside of his mouth for sores, just went through this with the two
young cats,
Antibiotics cleared it up but mouth sores lasted an extra week.
the old cat (19 y o ) never caught this he was the one I worried about.

Darryl

there are lots of possible causes for vomiting, diarrhoea, loss of apetite,
loss of weight, dehydration and crying is also normal in such situation (in
vet-terms called "acute abdomen"), you didn't give him painkillers I hope
(NSAIDS can cause acute abdomen by gastric ulcerations and hematemesis)

most common are of course the gastro-intestinal causes: in stomach:
gastritis (seldom with fever!), ulcers, foreign bodies (hair-balls); in
intestines: obstuction (you can have diarrhea and obstruction in one cat!),
ulcers; pancreas inflammation, peritonitis, splenic torsion, lower urinary
tract obstruction (you cat isn't neutered at very early age I hope?) trauma
or congenital defects leading to organ displacement or entrapment in the
hernia will also lead to abdominal pain if the vascular supply of the organs
involved becomes impaired or ischemic
also dietary intolerance or hypersensitivity, or infectious causes
(panleukopenia, corona, FIP, Salmonella, Campylobacter (can give by the way
hematemesis) clostridium, E.coli,..., parasitic infection, toxins)

risk factors are as usual exposure to NSAIDs or corticosteroid treatment,
garbage or inappropriate food ingestion wich can give pancreatitis, foreign
body ingestion which can give intestinal obstructions

tests that are usefull:
bloodcell count: leukocytosis! (high white bloodcells) are often associated
with inflammation of infection
                         anemia ?: can be seen with gastro-intestinal (GI)
ulceration
azotemia? is associated with prerenal, renal and postrenal causes
elektrolyte abnormalities can help to evaluate GI disease[Cl] and renal
disease [K]
hyperbilirubinemia and elevated hepatic enzymes help localize a problem in
the liver or biliary tract
TLI can be useful in evaluating feline pancreatitis
abdominal radiograph can show abdominal masses or changes in shape or
shifting of abdominal organs, but not all foreign bodies can be seen on
X-Ray; loss of detail is an indication for a peritoneal tap;
abdominocentesis is a necessar test

tell me about the tests that were done yet, so I can maybe help a bit

Erik (the vet-but you should have known that yet, hmm)

I mentioned to a group of cat lovers that my cat who was vomiting frequently
made the same grinding noise with her teeth when she ate.  Someone mentioned
she was nautious (upset stomach since I KNOW I didn't spell it right)  I
started giving her Pecid AC and it has stopped.

Still haven't figured out why she's loosing weight and WHY she's got an
upset stomach all the time.  Got yet another vet appointment to figure it
out.