He was casing the area for several hours that day before Trump started speaking. A counter sniper team on the AGR building roof would have been easy to spot. He would have seen the SS counter snipers behind Trump and noticed no such team on the AGR roof. Trump starts speaking and he sees his chance and goes for it. Could he have had help that day? Sure, but there hasn't been any indication that he did have help for now.
Time to forget everything you think you know about this assassination—and start over from scratch:
A good place to start is to scroll down until you see two counter-snipers ducking and cowering when the first shot was fired.
Why? Because the first shot was fired at them.