The correct answer is "I feel customers may think you have a poor attitude."
If the boss wanted to communicate more effectively by utilizing language, he would have to say that "You have a bad attitude with customers. I feel customers may think you have a poor attitude."
This is very important because probably the employee is not aware of the situation and the consequence of its behavior. That is why the boss has to tell the employee about the problem but also the reason why he wants that problem to be corrected. This way the employee is going to be aware that he/she has to change its attitude not only because the boss says so, but because its attitude is harming the company.
So language is very important to create understanding in the workplace.