Definitely go with Blackberry. They are one of the more reliable and easy to use smartphone and they never go out of style!
Try going with a Black Berry Torch!
-Comes with big touch screen
-Slide up BlackBerry Keyboard
-NOT like one big button like the Blackberry Storm!
The Droid Eris is practically free now. The Motorola Droid and Droid Incredible used to be $200 with new contract, however, they are probably cheaper now. I have the Droid Incredible and I love it. The Eris is a little outdated but, if it's for your kid, who cares?
Any of them will require $30/month for a data plan.
I would definitely make sure it has the Android operating system. If you get him a Blackberry, he's just going to complain that his friends all have iPhones.