The officer was absolutely within his rights to respond once the gun was flashed (yes...carrying concealed means carrying CONCEALED...when the weapon became exposed the officer was just dealing with a man with a gun). There was absolutely no cause to threaten to shoot him in the back and his language was certainly unprofessional. His RESPONSE was unprofessional. He SHOULD have had the man stand with his hands on the vehicle, cuffed him, removed the weapon, asked the man to produce his CCW, and then uncuffed him, returned his firearm, discussed with him his reasons for responding, issued the citation for expired plates, and sent him on his way. Could have been a pretty quick process.
If you are going to carry concealed you HAVE to ensure the weapon is concealed. And even if the law doesnt require it...its a good courtesy to hand the officer your drivers license and CCW IMO.