None of us know what really happend here. Im not even sure the father really knows.
If The Rebbe abused the son in any way, then it IS appropriate to talk bad no matter what, there is never an excuse to berate or engage on corporal punishment.
However if the boy did do the wrong thing and was appropriately punished (Doing the wrong thing does not mean the rebbe has the right to berate or hit for ANY reason) then he should apologize