If you want to teach your son to shoot with open sights, I would suggest getting him a .22lr. These are cheap enough to do a lot of shooting, with little noise and no distracting recoil. After he masters this it will be much easier to adapt to the noise and additional recoil of a centerfire rifle (mild though a .243 is.)

A .243 is capable of much higher accuracy than most of us can achieve with open sights. I'd scope it now.

Just my $0.02 worth.