Give your vet a call for advice. I applaud you for rescuing your little guy, but if you can't afford medical care for him, you'll need to try to find him another home. I hate saying that, but providing care is a necessary responsibility of pet ownership. Here is some info I found on another site:
What to Watch For
Gastrointestinal symptoms include nausea, loss of appetite, vomiting, diarrhea, melena (black, tarry stools), abdominal pain and ulceration of the stomach.