C++ is nice and a sure value, knowing it will pretty much enable him later to quickly grasp any other language in the family (Java, C# etc). BUT it is also arcane and complex. Some aspects of its syntax are rather difficult and unpleasant.
Python has definitive advantages here for the learner :
The REPL : To be able to whip up a console and try right away some syntax and get results is a very powerful learning tool. You get immediate feedback, which tickles more efficiently the brain's reward mechanism, thus making learning more fun for one, but even more important, makes lessons easier to remember.
Extensibility : It is easier to jump from python to another language because many of them are directly accessible from it. He can then have some quick expeditions in other languages while staying in the comfort of the already acquired knowledge. Jython, Iron Python or C++/C through CPython's natural extention mechanisms.
Cost : a lot of good material and good environments are available for free with Python. Though this is not limited to Python, it is certainly a cultural trait as the community has always been open and transparent.
On the negative side though is Python does offer a lot of line to hang oneself with. That said, with your supervision (I assume here you, yourself, are a programmer) you can help guide him towards the good practice.
Having said that though... the biggest influence would most likely be yourself !
Python (or any other languages for that matter) could be the best possible academic language, but it is useless if you cannot help ! So regardless of my opinion of Python here, if you feel more comfortable teaching him C++ because this is what you know best then C++ would most likely be the best choice for both of you.
a little something I stumbled upon that might prove useful : Pygamezine
This also might prove useful : Intro to Python ecosystem