For the exact answer to which one has the most felt recoil, you can go to the Beartooth Bullets website, and then go to their ballistics page, and you will find the formula for computing free recoil energy, based on the speed of the bullet, weight of the bullet, weight of the rifle, and grains of powder in the load. You just plug these in on the screen, press Enter, and the formula will give you the answer in footpounds.
From personal experiences with both calibers in light rifles, I believe you are totally correct. The felt recoil of the .357 magnum, standard 158 grain load, seems almost non-existent, compared to the recoil, noise and muzzle jump of the .243 Winchester in a light rifle.
This may be due to the fact that the .357 magnum is traveling at very low velocity, and the .243 Winchester is probably approaching 3,000 feet per second!
I do know that you if pay to put a nice recoil pad on the rifle, such as a Pachmeyer Decelerator or a Kick-Eez, then your son will definitely have no problem whatsover with the .357 mag, and in all likeliehood, he will have no problem with the .243 either.
Personally, I would start him with the .357 Mag, since it is much more fun to shoot and plink with, and its alot cheaper to in terms of factory ammo.
Just my experiences.
Mannyrock