I didn't love Albi (the church is impressive). I loved the area though. It's now my favorite part of France. I flew from Paris to Toulouse, rented a car then drove to Carcassonne and started north toward Bordeaux, then trained back to Paris. You could train from Paris to Bordeaux, rent a car and start south.
The Dordogne is beautiful but can be crowded. The Lot is a little more off the beaten path. Languedoc, except Carcassone and a couple other spots, is surprisingly uncrowded (or at least was uncrowded last June when I visited).
My favorites through the area: Carcassonne (but only went at night), Foix (great castle visit), Cordes sur Ciel (perfect medieval town but I guess gets crowded in summer), Najac (great castle and adjacent town), Pech Merle (actual cave art, not reproductions), Conques (way better than Rocamadour), Domme, Beynac and Castelnaud castles and Sarlat. I regret not visiting Narbonne. I also liked St. Emilion (the underground church was much more than I'd expect from a small town) but I think it can get really crowded too.
If your son likes to hike up a mountain to visit castle ruins, there are quite a few to choose from in Languedoc. I visited Peyrepertuse and Queribus. Puilaurens is another good choice. There's no admission charge, just park, hike up and explore. The mountains aren't huge but the castles all have commanding views.
You could drop the car in Perpignan and train to Barcelona or keep driving down the Costa Brava. If you drive, stop at some of the small towns along the coast. The beaches here are much better than the man made beach in Barcelona.
