First please note my very important edit to my post#10. My apologies.
There are so many ways to teach electricity but I feel it is important to follow the route that your nephew is following with some extensions if he has the interest.
Too many traditional physics texts start with a whole list of declarations which are only semi connected and appear to serve no real purpose.
To be studying parallel circuits at 13, perhaps your nephew's syllabus starts somewhere to provide a real motivation. That would be good.
But whatever you do, my advice would be to avoid Horowitz.
That's OK, I knew the convention. I think I need to get ahead of him a bit so I'll start learning it now. I'll try Khan Academy out.