It is proper to say "Can you check out for him first?" in subway?

In the lunchtime, I went to Subway to buy a sandwich. When I got in the Subway restaurant, there was only one customer, who is my physics teacher, and the worker was making a sandwich for him. In this case, I thought that the worker would help my physics teacher to pay his bill first, but the worker asked me what I want and made my sandwich for me, leaving my physics teacher waiting for about 2 min. I felt bad about that. I wanted to say something to let my physics teacher pay his bill first, but I did not know whether what I wanted to say is correct or not.

So, in this case, can I simply say "can you help him check out first" or "can you let him pay his bill first"? Are there any other words I can say?