Rat is dying help
let him go in peace
Well I reckon, just make the last days be the best day of his life. I'm sorry but it doesn't sound like he'll make it, but cheer up. Focus on the happy times with your rat. Maybe you can get another rat and name him J.R. There is something you can do. You can pray to God to help heal your rat.
Hi there. I'm really sorry for your situation.
Rodents do have very short lifespans. I can best suggest that you just keep him comfortable, give him soft, fresh foods like fruit/veg, give him a water bowl rather than have him have to struggle to use a water feeder and make sure he has access to a warm, dark place to sleep and keep him away from noisy places and don't overfuss him.
Make sure the room you keep him in is well ventilated and avoid spraying perfumes in the room where you keep him. Also, how warm is it in the room? Too warm? Is he getting dehydrated? If you have a reasonable climate, what happens if you take him (still in his cage) outside? Does he calm? Does he get worse? Make sure his illness is not caused by his environment! Rodents can be poisoned by chemicals like those used for cleaning, those sprayed in perfumes etc.
You haven't mentioned how old he is, or why you were taking him to the vet before.
If your mum cannot afford to take him back, try finding your nearest animal welfare charity/organisation and see if you can contact the owner to see if it possible for at least their resident rodent expert can give you some free advice. People who set up the local animal charities are genuine animal lovers, and may be able to help you, may even allow you access to a low-cost or free vetinary service.