RE: Caliber for son's first deer rifle?
When I bought my son his "first" deer rifle, I also bought him his "last" one. I got him a 30-06 and loaded it with reduced recoil loads. You can now buy them OTC, but at the time, we handloaded them.
Being on a limited budget, I felt that with the '06, we could get him a rifle that could be used for just about everything he'd be likely to hunt, have readilly available ammo (if he had to buy it), and would have a wide variety of bullets to use. The .243 will do fine for whitetails, but I don't care for the limited bullet weights.
I also am not a proponent of the modern, TV mentality that says that only standing, broadside shots are valid. Most of the well-known gun writers and hunters of previous generations advocate taking frontal shots and also "raking" shots that must pass through heavy bone or muscle to reach the vitals. For this, a lightweight bullet will not work as well as one that is 150 grains or more. If I could sit for a week in a blind with a camera rolling over my shoulder on a ranch with hundreds of deer from which to choose, I might be able to hold out for a standing broadside shot - but in real life, it doesn't always happen that way.
Just my opinion - but that's what was asked. Good luck, and have a great time teaching your son all about hunting.