unfortunately, there are a number of different health problems in cats that have weight loss as a symptom. some far more severe than others. it can be very dangerous to try treatments on a cat when you don't know exactly what's wrong with them.
believe me, i understand not having very much money. when worst comes to worst, i will use my credit card for vet bills. the most important thing here is your cat's health, and that may require cutting costs elsewhere in your life, but it is worth saving your kitty's life.
good luck!
Wow, may illnesses cause what you describe. Like Zodiacqueen said, without the tests there is no telling what his problem is.
Try feeding him with a syringe or eye dropper. Cat milk, kitten formula, goat milk...these are all good sources of fat, protein, vitamins and minerals to boost his system. Force feed him with a syringe and keep him warm and comfortable. Make him drink water. Hydration is very important. Again, an eye dropper or a syringe can help you accomplish this task.
Try to ask for payment plans. Talk to your local SPCA, search online for veterinarians in your area who will accept monthly payments. Ultimately, a credit card may be used since credit cards do have payment plans.
I'm sorry your budget is tight. I would fight to save my cat's life, and then, if nothing can be done, then I'd let him go; just don't give up on him if there is something you can do.
Good Luck.
Sorry to say this, but without the tests I don't know what to suggest. You've got to know what you're up against and ya can't just make guesses.
Keep trying vets until you get one that'll take post-dated checks, or Carecredit(special credit card for pet care only). Look this up on-line.