A man is reaching out to his pocket. The officer notices this and instinctively thinks his life could be in imminent danger as the suspect could be reaching for his gun. The right thing for the officer to do in this situation is to go get a warrant. Why worry about your own life, warrant is a must.
Does that make sense to you? I think yo are arguing that the cop should've gotten the warrant before he patted the suspect. That's not what I am discussing here. Most of us are discussing the officer's use of "excessive" force to subdue a suspect who he percieved as a threat to his own life. How is probable cause and reasonable suspicion relevant at that stage? Those two are relevant when the cop is approaching the old man, and they had reasonable suspicion part covered there.
If this doesn't clarify it for you, I think nothing will.