If he is permanent resident of Canada so nothing to worrie about, the officer will not ask too many questions, they might ask him these kind of question like, have you been travel to the US before, how long are you going to stay there, where do you working now, something like that. As long as he tell them the truth. However, if luckily they might let you in with him so you may help him for conversation but anyway he will be fine.
All the best