So on the plus side, on an iPhone, the text is big and clear, it's automatically updated when you access the page, it gives a graph of the stock and it's very easy and intuitive to use.
On the downside, it's expensive, tied to AT&T - and it's a touchscreen.
If you haven't seen it - this is what the iPhone stock page looks like:
http://z.about.com/d/ipod/1/0/...-/iphone_gallery_1.jpg
For the screen, you can get screen protectors - I'm using "Crystal Film" which I'm very pleased with. The screen is glass, so it's vastly less prone to scratching and other issues compared to iPods. But if his hands are dirty, then you are right, it might not be a good choice.
I spent a while trying to decide between a Blackberry Pearl 2 and a 4GB iPhone. They are both nice.