Another Texas suggestion.
Big Bend is a great suggestion, but if your son's High School Spring Break corresponds to one of the several weeks of College Spring Breaks, be prepared for "no room in the Inn" and long lines of college kids where ever you go. Two alternative places to stay in that area (where the college kids don't usually go) are <a href="http://www.terlinguaranch.com/about_visitors.html">Terlingua Ranch </a> (just north of Big Bend) and <a href="http://www.freeranger.com/stillwell/">Stillwell Ranch</a>. Stillwells, is primarily tent camping, but I think they still have some old trailer houses for rent. They are a bit older and more rustic, but the places oozes with history and it's a bit of a hoot to stay there, an is pretty cheap.
Another alternative to Big Bend is something I was involved in this last fall. The Western Trail was Texas' greatest cattle trail. It basically extended from Bandera, Texas to Dodge City, Kansas. It is the least developed of the historic cattle drives and it state highways across three states pretty much follow it's route. It can be driven in about a week and there is terrific history all along the route. I blogged a cattle drive re-enactment in the Fall and you can still see it at <a href="http://www.elderbobonthetrail.blogspot.com">OnTheTrail Blog </a> The site has links to a ton of historic information as well as thirteen audio interviews with a variety of experts regarding things to do with cattle drives or the towns and people along the way. This trail is very undeveloped but there are awesome sites along the way, and plenty of places to stay. If you need more information, e-mail me.
elderbob