I'd get him to write the selection criteria responses and then review. He's going to need to know them inside and out.
Get him to start thinking of examples. What are they going to be assessing him against? If there is a general capability statement for his level (for example the APS has the Integrated Leadership System) get him to start looking for examples that best meet those capabilities. Look at what is expected at level above and start seeing how he meets them.
If he had selection criteria when he applied for his job, revisit them, see if he could improve upon them.
Practice questions are important and making sure he is going through and clearly describing his examples - situation-task, his action and the results.
Good luck - I had to reapply for my job and it's super stressful on everyone. The people we were interviewed by had no background in our area which I found useful because it meant that I had to explain why what I did was important (if that makes sense).