Umm, a little confused. Do you have a rat or a rabbit because the thread topic says Rabbit needs help" and the contents mentions a rat? I read some months ago about a cat who had a large lump on her neck and the vet said nothing could be done. He didn't even bother to say whether it was cancerous or not or to do any tests. Anyway, she gave the cat 500 mg of Vitamin C twice a day to start and after about a week she increased it to 750 mg Vitamin C twice a day and in two weeks the lump was completely gone. That's the powdered vitamin C not the chewable or time-release . It comes in a little bag and it's a white powder. 1/4 teaspoon of powdered vitamin C(it's called ascorbic acid
when it comes in a powder like that ) equals 1000 mg . It's kind of bitter so it either has to be mixed with something and given with an eyedropper of disguised in food. So that's about the amount for a cat which is about the size of a rabbit. If you have a rat it would be less , like 1/10 of that amount twice a day. You could try liquid vitamin C that's available at petshops and add it to the water supply but the powdered vitamin C is alot more effective but it's a little more difficult to give because it's so bitter.
BTW, you might want to separate the sick one from the others. Don't know if it's the same for rats or rabbits but sometimes, when an animal is ill, the others will sense it and either stop it from getting to the food dish or try to attack it because they sense it's weak in some way, sort of a culling behavior.