I drive a Renault Scenic, and also suffer with a bad back. However, driving doesn't affect my back nearly as much since having the Sceneic. The seats are actually quite high, and for myself I find that the ability to position the back into a fairly upright position is ideal. DH complains it's like driving a lorry, but I find it very comfortable. Plenty of room to carry 'stuff' as well.
Pilates can be done at home - the Lynn Robinson DVDs are good. They're cheap and easy to use, and might help him be more aware of the muscles that are going wrong.
Alternatively, is physio an option? He might even be able to get some suggestions for what to change (or what to look for in a new car), to resolve the problem. I've certainly had help from physios about bike setup, baby-carrying, etc etc. (NB: physios are not alternative medicine - they train in hospitals and use science. In case this helps.)
Oh, and in the interim, one thing that might help would be changing settings mid-journey. Because almost any position gets uncomfortable after a while, and it might be the staying still that's the problem, rather than the car per se. So if the seat height or back angle or whatever can be adjusted easily in transit (ok, maybe at a light, not on the motorway!) that might help.
All good advice, the main hurdle has been encouraging him to do something at all, he's a bit of a cynic with this kind of thing. We need to think about changing the car anyway, and don't want to get it wrong.
MrsSprat the most comfy car i have driven is a discovery 3. probably because of the higher/upright position. ime the higher the seat the better. we had a c-max once also that was comfy. if your dh drives a lot it is worth getting the right car as well as physio.
I find my BMW 3-series very comfortable and supportive, and the seat is very adjustable. I'm not particularly tall though and although I do long journeys occasionally I don't do many hours in the car on a day to day basis.
I should also say that with previous cars I used to walk as though I needed a zimmer frame after a journey of 3 hours or so, but I haven't had any back spasms with this car. Also my car is automatic which I think is kinder to my back in town traffic.
I am afraid you are going to have to drive a swedish car. Saab and Volvo have great seats, based on airline pilots, and have most newer models generally have adjustable lumbar support, which is great. Well, my 2004 model does. Avoid Hondas - can't drive dh's for long journeys because it gives me chronic back ache.