Trigger time!! Stop obsessing about bullets and the minute details of expansion or jacket retention. If the bullet hits the wrong part of the deer, it won't matter.
So, when I was learning to hunt (which came mostly as an adult) I started with a 30-06 and missed a whole lot of times. I switched to a .300 WM and things got a lot better. Not that the rifle was the problem, but I needed more confidence in my field shooting. With the 300, if I can hold on the crosshairs on the body, the shot goes in.
I would suggest taking the young hunter to a safe shooting area and practising field shooting at various distances. Give him the opportunity to get some muscle memory and visual sight pictures of what different sized targets look like at "deer" ranges. Shoot up a hundred rounds if that is what it takes, but give him the trigger time to be a good field shooter. A holdover diagram taped on the butt has always been a good reminder.
For what it's worth, most of my Saskatchewan deer have been killed between 50 and 200yds, with the longest at 450yds (by GPS). I try to shoot a standing deer who show their full ribcage. Aim for the slats, and they'll go down, or run a bit and then go down.