There are a lot of possibilities. Dogs are often scolded when they growl, so they are less likely to growl and warn before they bite. They still try to warn, but a nine-year-old probably did not see anything going on.
It is possible that the child was playing too rough, touched a part of the dog that was painful or sensitive, and of course, it is just possible that the dog got too worked up and bit like he would another dog. Kids can be very fragile, so he may not have even meant to bite so hard.
The first thing I would do, if this were my dog, is to take him to his regular vet and have a thorough exam. He may have a painful bladder, early arthritic pain (joint degeneration), or something else that the vet picks up. If your vet does not find anything, and you are in a city that has a behaviorist, it would be a good idea to have a consultation.
If your dog were to bite again, the victim might win if they chose to sue you.