Can anyone please help me out with this?

I haven't been here in a while but my poor Cavalier Frodo is ill. It started two weeks ago when he would yelp if we just walked by him, or tried to put his leash on. It has progressively gotten worse and now he barely moves because he will start yelping and going crazy, holding his right leg up and sometimes walking in circles. We've taken him to the vet three times now (and racked up a ton in vet bills). At first our vet told us it was just a pinched nerve and that it would go away in two to three weeks, but this is week two and it only seems to be getting worse.

We researched SM and honestly the symptoms do not match up. He was not air-scratching obsessively and it isn't like he's going paralyzed. Here is one very odd thing - he lays around all day and will only eat when he put food and water right up against his mouth, but if we take him for a walk he seems perfectly fine! He will trot with me and use the restroom and sniff around, without yelping. It seems like its only when he's inside and sleeping that he will randomly start yelping and act like his paw/arm/shoulder/neck is hurting him.

He went to the vet the other day (again) and X-rays were done. The vet said he saw a mass at the base of Frodo's skull (which scared us, because we know SM has to do with the brain coming out from the back of the skull) and the vet also said he thought he saw two slipped-disks in Frodo's spine. He sent the X-rays for more observation and they came back saying there was absolutely nothing abnormal with them, so we are back to the pinched nerve theory.

As you can see this has kind of been a rollercoaster ride and we are still confused. He's on several medications for pain, and for his heart (he has a leaky valve, and is also going blind in one eye because that eye does not produce tears). Frodo is a mess, but this is just heart breaking and I am hoping someone on here knows of something we can try, or has been through something similar. The vet told us its either wait and see, or send him out of town for a cat-scan, which will be three to four thousand dollars and we cannot afford that.

Any information will be greatly appreciated!! We don't want to lose our Frodo. If it helps, he is of average age (not a puppy, not an old dog). I'm not 100% sure what the age is but I can find out. I would guess seven or eight.