My poor baby has red patches about the size of a quarter on his rump...I shaved all of his hair back then...

But has been itiching so bad....he is such a well manner baby.....he has not lost any hair, besides me shaving him....so I am not sure if it is mange....can anyone relate.??????...I am treating him with antibacterial ointment but I think we need to head to the vet...I feel so bad that he is not able to scratch his poor rump....