Is is possible that, rather than internal bleeding causing his stools to be black (and I believe it has to be high up in the gastrointestinal system to cause that, whereas lower bleeds just show blood in the stool) it could be all the flea dirt he has eaten? After all, that is partly dried blood - his blood. Probably wishful thinking on my part.

Is your rescue having a hard time getting donations overall? Can you put up a special drive on your rescue's web page? You are right, it could be a major expense if they are going to pull him through when his systems have been so poisoned. Do you have the name of the product used so you can give that to the vet? The shelter I work with is getting donations, but they are also getting lots of cats back that were previously adopted from the shelter, older cats that need more vet care, and their adoption rate overall is definitely down.