There is no reasonable range where a 7mm mag is needed for deer.
I wouldn't saddle a 16 yr old with the 7 mm mag either.
If you're stuck sharing those two rifles, suck it up with the magnum and let the boy hunt with the 7x57.
I would borrow the 7x57 back for most of my practice if I were you though. Just shoot the magnum enough to get familiar with it.