Your cat has several conditions that would be concerning if they were the only thing happening. Taken altogether I suspect your cat will not get better without medical attention.
Lower Stress Options for Vet Treatment
The least stressful way for a cat to see a vet is to have the vet come to your home. If you can't find a home vet for your area on the internet, call several local clinics to find out if they do home visits (or can recommend someone who does home visits).
If this isn't possible, in North and South America, you can look for a Cat Friendly Practice (other locations may have a Cat Friendly Clinic). This is a certification program to ensure these veterinary hospitals take several steps to ensure that the visit is as un-stressful as possible, including
- Each Cat Friendly Practice® makes an effort to have a calming environment.
- A waiting area or experience that reduces stress associated with other pets, or unfamiliar smells and smells.
- Cat Friendly Practices® can also help advise you on ways to reduce stress before and after the visit.
Finally, many of my cats feel better if I bring a blanket that smells like home for them to hide in. The vets that I've seen have all been entirely understanding (and some appreciative) that my cat was calm and relaxed hiding under the blanket, and would only expose the parts necessary for the exam as needed. We've even done a echocardiogram (ultrasound of the heart) with the cat's head completely covered by the blanket.
Listed Health Concerns
A cat who is breathing fast may be suffering from a heart condition. Untreated, the cat will slowly suffocate from fluid backed up into his lungs. Fortunately, there are some very easy medications that can make your cat more comfortable and prolong his life! The cat in that question could have died that day if we hadn't taken him to the vet. With medication, he lived for another 10 months after diagnosis.
If you can feel your cat's spine more than you used to be able to, he may be loosing muscle, fat, or becoming dehydrated. Only your vet can determine what type of tissue loss your cat is experiencing, and what the underlying cause is. If it's dehydration, this is a serious condition that needs to be treated quickly. If it's muscle loss that's still a serious condition, but can be treated more gradually. If it's fat loss, that depends on your cat's previous condition.
Any time your cat's eating habits change you should see a vet to make sure there's not an underlying health problem. The most common cause is usually one (or multiple) feline resorptive lesions. You can think of them like cavities that are SIGNIFICANTLY MUCH MORE PAINFUL. Gum disease/inflammation is also very common. Both of these conditions are treatable, but your cat needs to be assessed by a veterinarian.
If your cat ate a poisonous mouse, he would need to see a vet immediately. Many rodent poisons are anticoagulant (which means they prevent the blood from clotting). Ingesting this poison will cause your cat to bleed to death very quickly.