I'm also a 40ish man, about 5'11". I was having back pain, and it has stopped. While I haven't found a perfect solution, here's what I'm doing.

- Nowadays, I'm mostly using a kneeling chair.

- I've also built a button chair. (See the TED talk at https://buttonchairs.org). I use it some of the time, and it is okay.

- I also use a fairly plain wooden chair at times.

Additionally, I take regular breaks (every couple of hours) and go for a walk around the block. I've also heard that exercising your core helps.