Is your sons rifle labeled 223 Remington or 5.56x45?
The two are dimensionally identical but the 556 runs at slightly higher pressures, may or may not have less internal case capacity and the firearms sometimes have a longer leade, for heavier bullets. There is some disagreement about firing 556 ammo in a 223 but there is certainly NO trouble with shooting 223 in a 556.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.