I disagree with gtaus on this one. Many veterinarians will see rabbits, and if cost is a huge issue, there are bunny rescues out there, and vet clinics who see bunnies may have contacts that would be helpful.
Finding a veterinarian for chickens is much harder, and one big reason was mentioned...
If at all possible, get him to a veterinarian who sees rabbits soonest, to sort out what problems he might have. Internal and external parasites are very likely, given his previous lack of care.
Good for you for taking him in!!!
Bunnies are very cute and can be great pets, if not for you, for...