What events and experiences lead Bruno to gradually give up some of his innocence and see things differently?

I might consider the moment when Bruno changes his clothes and hops the fence into the death camp. Bruno has ceased to be in a state of otherness compared to Shmuel. He has disobeyed his father's instructions of ,"Out Of Bounds At All Times And No Exceptions."  He has also left the safe world of the German elite and entered Shmuel's world of the oppressed Jew.