Rascal is sick

Hello, I have two cats, Rascal (12) and Tyger (11). Both have been fairly healthy although Tyger has an autoimmune disease and needs meds occasionally. Rascal has'nt been to the vet in years and a couple months ago he vomited all over one night and the next day he was hiding behind the couch and would'nt eat so I took him to the vet. The vet said he was dehydrated and did a blood test and his WBC count was off the chart. He suspected leukemia or a bad infection and gave him a steroid/antibiotic shot and the next day he was back to his old self. I was supposed to take him back in a week or so for another test, but did'nt as I can't afford it. All was fine until last week. He threw up a little and started eating less. Now he's barely eating anything and won't drink more than a sip if that. He's getting very bony real quick and he was always on the heavy side with a little flab on the belly. His gait is very wobbly and his coat looks disheveled and I'm sure he's dehydrated too.
I'm not sure what to do as I'm sure the vet will want to do a bunch of tests and my financial situation is'nt good as I'm living paycheck-to-paycheck and had to defer my student loan payments again as I can't keep up (I'm a nurse, but only an LPN and I borrowed too much money and used too much credit cards going thru school and trying to make the house, car and utility payments myself as I'm divorced). After doing some research, I'm wondering if he may have diabetes and the vet missed it. I checked his glucose reading yesterday and it was 138. Not terribly high since the norm is 60-120 for cats, but he has'nt been eating much either. Any idea what may be wrong with him? Should I take him back to the same vet and see if he can at least have another shot or should I try a new vet (I heard this one is'nt the best, but the better vets are farther away)? Or should I think of putting him down?

Take him to the vet.
There are somany things it could be.

Should I take him to the vet I normally go to down the street or try a new one? I probably should have taken him today, but they did'nt have an opening until Thursday and are closed tomorrow.

Rascal did'nt look good at all when I got up today and he threw up some really nasty brown stuff that smelled awful. I called another vet in town and they got me right in. The doctor was pretty sure he had FIP with his bloated belly and he could feel a big mass in there. He said they could hook up some IV's and he could possibly last a few more weeks, but would most likely never eat again. He recommended I put him down and I agreed as I don't want him to suffer any more. He had the weakest, sorriest meows I ever heard today so I knew it was probably time. He was'nt the most social cat and had a mean streak and never a lap cat except for the past couple months, but he will be missed...

Sorry to hear this sad news. He is at peace now.