Awwwww poor little guy! My heart is breaking for you both. I think if at any time you don't feel like your getting anywhere with this then perhaps it's time for a second opinion. Clearly there is something wrong by all the things you say. First priority...Fluids and then food. Good Luck, please keep us posted!
What are your cat's kidney and liver function values? Are they normal? And, has an xray been done?
If everything has been done and tried, it's time for some further investigation. I would suggest getting an ultrasound done. That is really your next best step to take. Although, it's expensive($300+), I think it's important to do at this point.
In the meantime, try everything in your power to keep him eating. You don't want fatty liver disease on top of whatever else is going on. I would try some chicken broth, human tuna, and some strained meat baby food. That's what I have in my arsenal when my cat refuses to eat.
Good luck and hoping the best for you and your kitty!
Your poor kitty. And poor you! If he has not been eating for about three weeks, I am quite surprised your vet did not administer fluids while, either subcutaneously or IV. I do hope he is getting liquids. If you need to give him liquids by dropper, put the dropper in the corner of the mouth and just dribble it in,a little at a time. Don't squirt it in all at once or suddenly, as he may aspirate it into his lungs, and you'd end up with a bigger problem than you have now.
Perhaps, the food is too rich for him right now, especially if its canned. You may want to try a little bit of chicken baby food or Nutrical (vitamin supplements). If he won't lick them, put a small amount on his upper lip and he will lick it off.
Good luck.
oh my this is beyond what we would be able to advise you here, we aren't Vets only pet owners as yourself.
I do question the Vets use of Metacam though, it can lead to serious side effects including what you are trying to remedy in the first place.(vomiting and lack of appetite)...I will give you a site to research all the meds kitty is on. hopefully you will soon get some answers that will lead to some help for the poor little fellow. I also wonder if stomach xrays have been done to rule out tumors or other obstructions?......
http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=1752