Unless the guy touches the firearm it is "legally" considered only a transition from concealed to open carry (not to say the he has a CCW, of course).
If you take "action" without the guy pulling the gun on you, you are,from a legal standpoint, the aggressor.
I would walk away.
The way laws are nowadays,and without a witness,you would likely find yourself in a pool of shit.
What are you going to tell the cops when they arrive to the scene and find the punk shot dead,with his gun still in his waistband and his friends (and likely a terrified store owner) testifying against you?
And what are the cops supposed to think?
It is a very thin layer of ice on top of the shitpool...