I had two operations on my back in the lumbar region and and missing parts of some disks.

The first thing they do when you go to physical therapy afterwards is put you on a stationary bike!

Issues with my back are diminished and/or eliminated with cycling. Everyone's different, of course. I was really afraid the position on a road bike would be hard on me, but it's not. I'd certainly keep how it will feel to his back in the back of the mind, but I wouldn't buy a bike based on position of the back. No matter what bike you get, the back is supported by your arms. In a more upright position, your back is doing a lot more work keeping balance, I think.

Karen