in the future, dose the cat before they enter your home and keep the cat contained to an area where fleas can't exchange hosts. i dose my strays at least a day before i move them inside to be fosters.
for extreme cases, frontline can take a few days to really work its magic.
your cat does need to see a vet. what you can do is call local shelters and see what they can do about getting you free vet care {or really, really cheap care}. another thing you can do is try to find a foster. go to alleycatallies.org and find the link to their feral friends network or post him...
