If a doctor suspects that a child's injuries are due to abuse, to whom does he report the injury?

Child protection. Any hospital er will have the number or if you have a local community help number for a child abuse protection agency would be contacted.
CPS. The laws are a bit different from state to state, but the usual course of action is to report any suspected child abuse to child protective services. Also, the healthcare facility may have protocols to involve social worker, case manager, and/or child life specialist. Http://www. Childwelfare. Gov/pubs/reslist/rl_dsp. Cfm? Rs_id=5&rate_chno=w-00082.
Texas law. Texas operates a hotline (from Austin) where all reports are gathered, tagged for acuity and referred back to regional DHFS centers for action. Texas law requires all those who regularly come in contact with children to report suspected abuse or neglect, The law protects those who make reports in good faith from any liability if the investigation shows no problem.