I have not had a student (I would love one though!) but I have mulled this over and here is how I might handle it! A good old chat about assessment! Discuss school policy and ask about what he knows about assessment through his uni course. Casually bring up marking and how you manage to keep on top of it - ask him how he is managing it? Maybe he does a lot of verbal feedback and if this is the case tell him to invest in or make some "verbal feedback given" stickers. It might come up (and if he is a good guy it probably will!) that he has found keeping on top of the marking hard (we all do) and then that gives you the chance to emphasise the importance of marking for assessing and evaluating the children. I am not sure if you set targets for students but this would be a good one for him to work on. Hope you get it sorted, keep us posted!