I have a stutter (genetic). I would hate for anyone to finish my sentence or assume what I would say. I do weekly trivia at this one restaurant and they got used to knowing my food and drink orders, so for that, it's fine since they know me well enough. But generally speaking, be patient and wait for them to say their order on their own time. Out of curiosity, does he order the same thing every time? Maybe you could ask him if he wants "his usual" and wait for his response. Then confirm it after you put it all in to make sure he knows you remember.