My cat won't eat - standard vet tests negative

[rcgeek25]

We have a 12-yr indoor shorthair male cat. About 3 weeks ago he started vomiting phlegm (he urps a lot, but usually because he eats too fast). The vet gave him a shot of cortisone (he is allergic to some of the furniture in the house), which helped the phlegm. About a week ago we noticed that his breathing was rapid and shallow. We took him to the vet again, but they couldn't find anything. His breathing has since returned to normal. He was very lethargic as well. He started throwing up everything he ate, then stopped eating altogether. We have taken him to the vet daily. X-rays show no abnormal masses, bloodwork didn't show anything (white cell count was normal), ultrasound of heart was normal, tests show no signs of kidney failure.
He has lost 3 lb in a week, and last we took him to the vet was dehydrated. The vet showed us how to mix up wet food and feed him with an oral syringe. We have been feeding him 3-5x/day with the syringe, plus some water. He does not throw up the food we feed him by syringe, but still won't eat on his own (we've even tried tuna, to no avail). He will drink water on his own.  
He has a bit more energy now, and will move around the house a bit, and his breathing is normal. He will purr if petted.
We doubt the food was tainted- our other cat is his sister (indoor shorthair, same age, no issues), and they eat the same food. Plus she is more likely to eat random strings, plants, etc, than he is.
If he doesn't start eating we will be taking him to a feline internal specialist this week.
In the meantime, does anybody have any ideas what this could be?  We are doing everything we can to get our furry buddy back to good health.

[opus88]

sorry I meant to include this link for FORLs.....

http://www.dentalvet.com/vets/cats/feline_dental_pathology_and_care.htm

[opus88]

hi and welcome.....it is so distressing when our little ones are ill and no reason can be found, I sure do hope you can get some answers soon and get kitty back to feeling well.

I am thinking 2 things....have his teeth ever been examined? there is a condition called FORLs that is very painful but xrays are usually the only way to diagnosis this b/c it occurs under the gum line... they sure do cause lack of appetite and lethargy.

http://www.petplace.com/cats/how-to-tell-if-your-cat-has-dental-disease/page1.aspx

also pancreatitis...I will send you a link to that as well. please keep us posted
we wish you all the best...post anytime if we and we can try to help♥

http://www.cat-health-guide.org/feline-pancreatitis.html