I am the author of nxtprograms.com. 5 years old is quite young, nice to hear that he is doing it (with some help)!. As you can see, I rarely but occasionally resort to words to help with some of the steps on nxtprograms.com, but from what I can tell, most younger kids can do it anyway without the words (or with a little help from the parents to confirm what they already figured out!). I use my 8-year old as a test case, but he is very good with LEGOs too, so I try to make it easier than he would need.
One challenge you have touched on is that the expectations of younger kids for what the NXT set should be able to do (e.g. sophistication of the remote control) is often higher than that of older kids, who understand better that this is a complex kit that is designed to require programming to get anything interesting to happen (and even then, pretty simple things), not a remote controlled toy that you build. The capabilities of a modern remote controlled toy are amazing nowadays, and they are hard to "compete" with as far as expectations go. If you are looking to build RoboSapien and the like with your NXT, you will be disappointed... Neverthless, I design a lot of simple remote-controlled models for the web site, because they are more fun to play with without needing to do any further programming. Unfortunately, limited to only the sensors that come with the base NXT retail set, you can only do so much.
LEGO clearly has a marketing problem with this. Look at the picture they put on the box (android). Younger kids think this looks really cool, but many are actually disappointed once they build it. And a lot of NXT sets go sadly to the shelf soon after that. Then many older kids start thinking that LEGOs aren't "cool" anymore, around the same time they start listening to Nickleback on their iPod...
Personally, I don't think it's so much a matter of age as the engineering mindset, that determines whether someone thinks the NXT set is cool and fun, which is something that some kids have and some kids don't. When I was in college, we were tickled pink when we could get the number 7 to appear on an LED display, because we were engineers. Our non-engineering friends would have thought we were nuts...
Anyway, I hear your request for printed instructions (a book), so that your son doesn't need to be on the internet, and I will be looking into that in the future. I would be interested in your (and anyone's) feedback on which of the following you might be interested in:
1. Full color book with all instructions, even if it was, say $40.
2. Black and white book with all instructions, with a reference to the web site if you have trouble following some of the steps due to print quality.
3. Black and white book with all instructions plus a CD with all the color instructions.
4. Small color book with instructions for a few models, plus all models on CD.
5. A way to easily and legally print the instructions on your own printer, including access to any updates on the web site as they arrive.
6. CD only, with color instructions, basically the web site on CD, no internet required
7. Software program on CD designed for kids with kid-friendly interface to full color instructions.
Thanks for your input (and anyone who wants to chime in) on this !