Here's my primer on how to check a fit:
https://www.bikeforums.net/21296948-post3.html
Fit doesn't have to be exact, but the above guidelines should prove helpful, just to see if anything is obviously amiss One thing not mentioned there, but useful in this case, is to check that he is not rounding his back. The back should be more-or-less straight from the top of the shorts to the base of the neck.
The real issue, as has been stressed above, is fitness. Many people think the perfect bike fit will eliminate pain. It will not. The only thing that eliminates pain is perfect fitness. And even that doesn't work forever, just take a google and look at the faces of RAAM finishers.
How to get the back fit? The best way is to ride lots. Ride until it hurts, rest it for a day, then do that again. After not all that many repetitions, the pain won't start for many more miles. That can be eventually increased to 100s of kilometers.
Of course off-bike exercises help too, and will shorten the time to fitness, but nothing makes your back as fit as cycling, lots of it.