If Martin was on top of Zimmerman and Zimmerman was the one calling for help, he is clearly innocent and the charges should be dropped ASAP. If there is some evidence to the contrary, it should be disclosed to Zimmerman’s attorneys, and, preferably, made public.
The witness named “John” has stated that it was clearly Zimmerman who was shouting for help. There’s no disputing that, unless of course you think John is lying.