I am sorry to hear that your kitten is having problems with his back. I know you said you don't have a lot of money to spend but I do think it would definitely be worthwhile to have him at least checked out by a veterinarian. His paralysis could be due to many reasons such as a past injury, neurologic disease or an infectious disease like the feline leukemia virus. If he did have something like feline leukemia, it would be best to know now (hopefully, its not the case). My other concern is that if he can't use his back legs, he may also not be able to urinate or defecate completely and this can lead to problems if not addressed. Some of these cats often need assistance with these two things and your veterinarian can show you how to do this. Most cats that are "paraplegic" learn to do very well. They often will get calluses to protect where their rear legs rub. The best you can do is provide a safe and accessible area that is easy for him to reach food, water and his litter box. Please keep us posted on how he's doing.
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