Any ideas on where to look for basic number theory concepts?

My younger son and I have reached the end of Art of Problem Solving’s Introduction to Number Theory book and I have a strange problem – he loved it.

I mean LOVED it. When we ran out of problem to work on yesterday, he was almost in tears because Khan Academy didn’t have any problems on modular inverses. Ha!

 

Anyway, he’s begging to do more, but I’m not aware of any other book on number theory that could even pretend to be accessible to young kids.

I’d planned on diving into the 2nd half of Art of Problem Solving’s Prealgebra book when we finished this book, but it isn’t as though I’m in any sort of hurry. I’d love to spend another month or two talking through more number theory ideas with him, but I really don’t know where to look. A quick check for number theory resources on Amazon, for example, brings up stuff like Hardy and Wright. Not exactly what I’m looking for 🙂

Any ideas on where to look for some fun and basic number theory?