If your guy can justify/articulate why he put a gun to someone he detained for a traffic violation just because he is armed, then there is no problem. If I was shot at or shot in the past, I may be like that myself, but since I haven't, I would just tell the driver I noticed he was armed. I would then tell him not to put his hands anywhere near it while I'm doing business with him.
P.S. I was shot at before, but not on a traffic stop. But again, if I'm happy with my level of safety on the stop, I wouldn't escalate the situation with a gun to someone's face with just the information you posted.