It depends on how often he uses his phone and where he backpacks. Being in a low-signal environment will eat up a lot of battery even on a feature phone plus having screens 5x bigger than feature phones also lead to more battery usage when being used. My old OnePlus3 which at this point is only used for alarms and has apps set to auto update (doze is on, but battery saver is off) in the background lasts around a month before I have to charge it again.

Smartphones also have 5x more screen to light up, too.

Reminds me of YotaPhone 2 eInk display - pretty crummy, but the Snapdragon 820 device stayed on a week on without additional power saving...

My old Turbo 2 could, in fact, sleep for a month in airplane mode (with crappy apps removed). However that would quickly drop to about a week with light usage, e.g. connecting to WiFi somewhere to get directions or pull email for the day.