Back support.

I use one of these:

Cool Mesh Back Lumbar Support Vent Cushion Car Office Chair Truck Seat Black

They're inexpensive and let your back breathe, so they're comfortable to use. If it doesn't work, then you're only out $5.

 

You're welcome, hope it works out for your DH.

FYI, you don't need to use the elastic to strap it over the seat back. Just place it on the seat and sit against it and it will stay in place. Then, if you switch out to take over driving for a while, you can easily move the support over to the passenger seat.

 

One simple change I made, which was a huge improvement, is to remove the headrest, turn it around, and reinstall it backwards. This is a super-easy, no-cost fix you can try. It's not perfect, the lower seat is still quite stiff, but it definitely stopped most of the back pain I suffered when I drove my Prius.

 

Xliderrider, thanks, cool mesh ordered. I've turned the headrests around. Sitting in my Prius has also triggered hip bursitis, (!!) anyone got any tips for that? (Other than get out and walk?)