Thanks for the kind words everyone! I'll ask my son whether he wants to release the source code.
He started learning with the Scratch programming language developed by MIT, which is awesome. But he's really wanted to have a standalone app on ipad for ages. I thought it would be impossible (objective c would have been really tough, I think!) But python was a shallow learning curve and programming on the ipad really helped.
I had to help with a couple of bits but I was surprised how easy he found it. The tough part for him was understanding object orientation, but we found we could handle it by just building on what pythonista provides by default for a Scene script (a godsend!). Then my role was simply helping out when he got lost in the syntax and compiling it as an app when we were over in Xcode.
It really is fabulous that he can develop and release on a tablet like this. Incredible to think my son's app appears on the same store as all the rest of the software he uses! Also, he's been really inspired by getting itunes connect reports about all the countries it's been downloaded from.
Simon