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 The Eris is a little outdated but, if it's for your kid, who cares?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.