My son was bitten by a dog last week. can a hospital refuse to admininster immunoglobulin?

Depends on dog: If there was good information about the vaccination status of the dog, then administration of rabies IG may not be necessary. The IG has risks and is quite expensive, so using it unnecessarily is not acceptable. Asking if they have a specific protocol if the dog's vaccination status is unknown is the best next step.

Wrong place to ask: In the US use of any anti-rabies globulin is based on best information available and often depends on local rabies activity, dog vaccination status, etc. Here a hospital doesn't decide whether to give this, the doctor treating the patient does. It is not given by hospital personnel because you ask. Practice may differ in your country & for that answer you should talk to local officials.