Any hints for things I should cover before my son starts his NICAS level 2 course soon. I'm no expert myself, but have taught him to belay sortof-OK (bit inconsistent use of right hand when grabbing the rope to do the 'hand-over-hand' bit. Otherwise, he knows how to tie in both as a climber and belayer. And he's experienced as a climber. What else should he need to know?
I have run a youth group that has taken dozens of young teenagers through the NICAS scheme. They all knew nothing before hand. I think you have already given him a good head start, anything else should be taught on the scheme. From memory, NICAS two is just about getting experience on the wall. Nothing much more technical than what he knows.
NICAS two builds upon the skills from level one and allows time to practice and consolidate these without introducing too many more skills. As long as he can fit a harness correctly, tie in, belay a climber, climb to the top, descend correctly and show an awareness of hazards within a climbing wall there should be nothing to worry about.