I told him that I knew he was frustrated by these particular things but that
they simply weren’t going to change, that they were inherent parts of the
job, and that I didn’t want us to be constantly battling over them … and
that rather than trying to force himself into a job that obviously was
making him frustrated and stressed, I wanted to see him figure out if he
could really be happy in the position, knowing that the things he was
complaining about weren’t going to change.
A couple of days later, he told me that he had thought about it and realized
he should move on.
So, in other words, you fired him. You made it clear that the job wasn't going to change, and that he didn't have a place in your organization. The fact that he submitted the resignation, rather than you submitting the paperwork is immaterial to the fact that you fired him. This is as much of a firing as a government minister "submitting his resignation" in order to "spend more time with his family" after a particularly embarrassing political scandal or gaffe.Now, granted, I agree that the firing was handled well, and was done with a minimum of pain to both parties. But you still fired him. I'm worried that OP is trying to sidestep that fact and make it seem like this was some kind of voluntary meeting of the minds when, in my reading of the situation, it clearly wasn't.