Start with simple tasks on real jobs. Even if he just has to watch you do certain tasks the first time. You hopefully have lots of simple and even non electrical tasks for him to do also, like carry in tools and supplies, drilling holes, punching holes, cutting and threading pipe, sweeping up, digging trenches, prepping luminaires or other equipment for installation, etc., and of course pulling conductors and cables.
After he gets the hang of some of this and decides he wants to continue to learn the trade then you maybe even look into some books and introduction to NEC and how to use it, maybe move on to a little more complex tasks like bending conduit, understanding simple switching circuits like single pole and three way switching methods, start emphasizing the need for safety regarding electric shock and arc flash hazards somewhat early in the learning process, find stories, pictures or videos of real accidents to show him just how dangerous electrical energy is.
I don't know that I would waste much time doing non productive installs on a training board in the shop, close supervision when learning a new task is still accomplishing some productivity, just don't try to push too many new tasks all at once or he could get lost somewhere. Repetition of new tasks is more helpful for learning than just doing something once and then moving on to something else. Let him watch you with some of the one shot tasks and try to assign him more repetitive tasks early in his learning experience.
