I'm making a similiar transition from kayaking to rafting, and the skills seem like they are going to translate pretty well from kayaking to oar rigging. I would start on any class III run he's familiar with and purposely take some less than ideal lines. For me the hardest part is figuring out what the boat is capable of and how its going to react. The rafts seem really survivable to newbie mistakes, but I dropped into a small hole sideways to see what it would do and almost had my first flip practice.
Running a paddle crew is completely different - tried that last weekend and I wouldn't have blamed the crew if they staged a mutiny.
I'm also thinking of taking the raft out to a lake or big eddy, throw it in upside down, and see how hard it is to flip upright. I know in theory how to do it, but I think my confidence would be higher if I've actually done the drill once or twice.