Shadey, I have a sofa with sort of a similar back. It came with pretty large 26 x 26 (Euro) style loose cushions. They overlapped each other a bit. What I did was to order 20x20 softer cushions with zip-off covers so that the back was softer.

I think a wedge shape would be firmer and not give the kind of look YOU like, nor would it give the comfortable sink-into feeling that your DH is looking for. Here is a shot of the sofa we have, sort of a British West Indies style. I like it because I can whip off those covers and wash them, and I can also make easy slip covers for myself.

In this pic, I had a blanket on the bottom cushion because of our two small dogs. That bench style cushion also zips off for washing, and is the same fabric as the back cushions.