It depends what he wants to do - if he only wants to read books - get a plain e-reader.
But, if he wants to do anything else (movies, games, internet) then I think it would be better to bite the bullet and get and iPad (or some other tablet, though I know nothing about them!).
Otherwise, I suspect that in a year or so you will be wishing you hadn't spent a couple of hundred dollars on a Kindle when you could have a tablet with more features.
Personally, I'm in love with my brand new iPad mini - perfect size for reading but great for movies, internet and just about everything else
But, they are more expensive, so you'd need to balance out your budget vs the functions you want.
Edited because I just noticed that he has an iPhone - in that case, I would either get a standard e-reader, just for books, or an iPad mini. I wouldn't get a tablet that uses a different operating system to the iPhone - there is no way I would want to use an Android tablet when I already have music, movies, apps etc setup for iPhone.
And, even though I have an iPhone, the iPad mini is just so much better for doing almost everything except phone calls (and when I get round to playing around with Skype, it might even do that!).
Edited by Rubybelle, 03 December 2012 - 12:26 PM.