3 year old Boxer

Question
My brother has a 3 year old pure bred boxer that is aggressive towards other dogs, strangers including children.  He has snapped at several people and has bitten one child.  We have contacted several rescues and do not want to have him put down, but none of the rescues will taken him due to the aggression.  My question is, can this behavior be corrected?  We live in Northern Kentucky and have not been able to find any help.

Answer
A 3 year old Boxer does not suddenly become aggressive. What has happened for him to be aggressive?

Was he always aggressive and now it is worse? Was he ever socialized? Has he had more than one home? Was he a rescue once? Has he moved from his only home he's ever known to a new one? Is there a change in the household?

I needs more information and history in order to access the behavior. Please provide everything you know.