Not sure if you know this, but the latest iPhone SE still uses the iPhone 8 body complete with home button. Depending on which phone he's on, it might be a good idea to upgrade; all of the rumors that I've heard are that Apple will make the next SE on a different body.
I know because one of my previous iPod Touch devices had a broken physical home button so I used the assistive mode sofware button instead.
This might also work for people who are too old to understand how to use the phone without the physical button.
Changing to the newer iOS was challenging for her due to the significantly smaller and pushed to the top call answering buttons. Apple does a lot for accessibility, but I really wish they had an “old persons mode”.
In the meantime, you should be able to solve the call banner issue through a relatively hidden setting; Settings > Phone > Incoming Calls > "Full Screen" should put things back to the way they were.
[0] https://www.apple.com/newsroom/2023/05/apple-previews-live-s...
I really dislike the little banner thing and much prefer a full-screen image of whoever's calling.