My first high power centerfire was a 270 (it was okay)....
My second was a 338WinMag (now you're talking)....

Nice to see the kid thinking outside of the box,
If he handles the 270 and the 3.5" Turkey Loads, then the 338WinMaq should be no problem.

338WinMag is a pretty gentle "magnum" IMO.... little more push.... little less "jab",
Make sure the easy fundamentals are taken care of:
- Pachmayer recoil pad
- Proper stock length fit
- Proper eye relief (both in the mounting and in the scope's eye relief length.... Leupold shines here)

Then, for the beginning....
- Not the heaviest loads
-Careful introduction --- standing and off hand [option, try iron sights, before the scope goes on]
-Early "buddy loading" exercise (sometimes live round, sometimes empty chamber) to ensure smooth trigger pull.

Just a reasonable approach to getting in tune with a more powerful tool, no big deal.