Suspicious Weakness/Starvation in Middle-Aged Chihuahua

I got a neutered Chihuahua who's almost eight years old. I've had him since he was a few months old. He's a very reserved dog who's never attacked me. No past experiences of abuse.

His appetite, up until recent months, had been normal. He's never been overweight, but he still ate often.

It all began about last year with me noticing him struggle a bit to climb up the stairs of our old house. This was usually after a good walk. My other dog is the same age and is a similar breed, although he's far from vulnerable.

After selling our house several months ago, we relocated to a temporary apartment in another city. After a few weeks, I started noticing his health taking a nosedive. He was refusing his dog food. I'll admit that I made the mistake of purchasing a cheap brand, but even the other dog still eats it just fine. In some instances that I fed the dog in question a handful of dog food, he'd sometimes throw up. A few months later, we moved back to the city that we used to live in. We found an apartment that was a few blocks away from where we used to live. Even then, he's still not doing great. In fact, I'm already noticing outlines of his ribs. He's not even looking forward much to walking anymore, although he still goes out for a bit to urinate and defecate. Rest assured, the stools are generally normal. He's had diarrhea in recent months, though.

I assumed that there was an underlying health issue, but just in case things got any worse, I went ahead with vaccinating him and the other dog. This was a few days ago, but he's still showing symptoms of tiredness and lack of appetite. The other dog didn't even show any symptoms. I'll admit that I haven't really been too involved in vaccinating them and always checking with the veterinarians, but my only excuse is due to having a tight budget. For personal reasons, I'm not employed and get very little money from programs that distribute them. I can barely buy myself a good pair of jeans with that kind of money.

Anyway, I'm currently at a loss as to what to do for my friend. His hair has been graying since a few years ago, but I don't think that it's simply age. Again, he's not even eight years old yet, but he's definitely appearing ill. I've considered donating both dogs to local shelters, but after hearing just some of the inhumane practices, I'm keeping them until it's time for them to go. I also don't want to risk giving them to potential animal abusers.

Even then, I'm still screwed. I don't want to put him down, since he's still showing that he's not yet done with life. I can see it in his eyes and the way his ears jump up that he just needs proper medical care. The best veterinarian with the best prices has asked me to consider a blood test for him, which will cost me nearly two hundred dollars. I don't have that kind of money, especially after breaking the bank to get both dogs vaccinated and the other one with some antibiotics and medication to relieve his itching.

What should I do? I'd appreciate all kind advice from the fellow dog lovers on here. I really care about that little piece of poop, so to even imagine having to put him to sleep would be a nightmare that would last a whole year for me. Both dogs are my pride and joy, and they've done only so much to comfort me in the last several years of depression and loneliness that I've personally experienced.

Many thanks.