*Yes,it is the right etiquette.But you shouldn't exactly pretend like he doesn't stutter,you should acknowledge it and make him know that you do and it's okay.It will make it easier for him because it will release some pressure off of him.How you should go about it - it is your choice,but I know it's easier for me when people trigger the 'talk',and then I can explain to them briefly that I'm stuttering and what it is,and what it is not.It takes really small amount of time,but it helps the stutterer immensely.Also,besides looking in the eye and not finishing his sentences,don't give him any 'relax,take it easy,breathe slowly' crap because that doesn't help either.Good luck,and huge props to you for actually taking time to make sure you're doing the right thing!* **EDIT: Yeah,I misread OP,guy is a customer,not a coworker,and triggering the talk would be confrontantional and possibly horryfing for the customer.**