Cat with various issues...and the vet "doesn't understand what's going on"?

I am sorry that I do not have true advice other than it sounds like he is dehydrated at least a bit. I would get a second opinion. I had a kitty who went through the bladder crystals, and he successfully went 7 years after with no problems, but an office cat of mine got them at age 6 and had to be put down :( The whole crystal thing seems to have no real rhyme or reason. Just want to let ya knwo your not alone! Good luck to you and Frankie!!!