How about if you call the vet, explain the situation, and then ask if you can volunteer sometime to work off the bill that is in excess of what your dad will pay?
We just took in 2 farm kitties - very undernourished (one was 2 weeks ago; one was this week.. ) Both had runny noses but went to different vets... one vet gave an antibiotic to the 4 week old,,, the other gave a nutrition boost to the 6 week old and said to give it time to run it's course. Antibiotics have their own side effects in kitties. Both are doing well now they are building up their little bodies.