I'd say the best thing to do is teach him how the camera works then load it with film and let him take pictures.
Teach him how to develop, post a recipe chart on the darkroom wall then, again, just let him go at it.
Same thing with the enlarger. Set him up and let him print.
Yes, there will be spoiled film and fogged prints but that's to be expected. Mistakes are part of life. Embrace the mistakes as "teachable moments" and just keep moving forward.
Then, when you talk about the finished product be sure to tell him WHY one photo is considered better than another.
Say something like, "I think this photo is better because the subject is more in focus." Or say something like, "This photo could be better if you added some exposure." In other words, keep the focus looking forward toward improvement, not backward to dwell on mistakes.
It's okay to make the occasional comment like, "This isn't one of your better photos," try to stay away from saying, "That's crummy," unless the context is right.
One thing that adults forget about teaching kids is that most learning is experiential. You can read about things in books or you can write things on the blackboard but most things aren't truly learned until the student actually does them. You and I have taken hundreds of photographs or even thousands. Your son has taken only a few dozen. If he had your experience, if he had taken as many photos as you have, there would be no point in learning. Would there?
The bottom line is to let him gain the experience. Once he has learned the basics, your job is to be the guide not to be the Lord and Master.
Then, one day, if he comes to you and says, "How come I can't make this picture look like one of yours?," you know you've got him hooked!