he is going to spend at least half a year every year for the next 5-6 years travelling Thailand before settling down.
Regardless of what you, I or anyone else thinks or believes, it's his decision and he personally believes and thinks a bank account will be very beneficial to him during those visits before he finds a place to settle down.
history is history regardless of time frame. If you've done something wrong or have been seen to be doing something wrong in the past such as overstaying or too many exemptions they will still grill because in their eyes, what's to stop you doing it again?
passport being new or not doesn't make any difference, a brief overview of your entry and exit history shows up on the screen when they scan your passport which they can then either look deep into your history on the computer or check your passport.
It's also not totally random, if you have an extensive history of exemption or TR use or even an overstay you're likely to be pulled out and grilled.
No, most people will tell you that airlines will ask you and immigration could ask but are unlikely unless you have an extensive history of visa exemption, overstaying etc etc.
If you're being grilled you've likely got an extensive history of exemption and/or TR use, you've overstayed or been pulled out and grilled before and now flag something up in the system when you try to enter.
he won't be retiring here for another 5-6 years. During his yearly 6+ month visits he will look for somewhere he wants to settle. Once he actually decides on a place or retires in that province he will open an account their but until then he just wants a bank account for QR, prompt pay etc.