It occurs to me that even a naive teenager would realize that with winter approaching, it is not a good time to head to Alaska. Chris McCandless did spend some time in the Sierra Nevadas. Could Blaine have gone there? Or, considering his car was last seen in Louisiana, could he have gone to the southern Appalachians? It's an area that doesn't get too cold in winter and where he may believe he has a shot at living off the land.
A couple of ideas: You still have his laptop, right? I wonder if you can get it forensically examined and see locations he may have googled. Or perhaps with the aid of your local PD, you can get a search warrant to examine his Google history. They do this all the time in criminal trials, but I don't know how easy/timely it would be to get one in this circumstance.