I think the 250 trailblazer will fit your needs very well overall. While the Blaster and 250 EX will certainly appear to fit your son better, I do not believe they are as stable as the trailblazer, not to mention manual transmissions on both, the auto is what makes the Trailblazer so attractive, remember, the wife wants to ride it too.
As for the purchase, I would definatly look for a nice used machine VS new. Many people buy these machines due to there good value, but quickly out grow the narrrow powerband and upgrade to something with more power. Re-sale on the trailblazers are not all that great and peaple often take a big hit, If I were you I would look for a nice 2000/2002 bike on the used market. You will be getting a much better deal.
The is only thing you will be looseing, in 2003 the trailblazer got the long travel suspension (same as scrambler), but like I said before you will probably out grow this machine soon and want something else anyway.
Good luck