If your son's phone is a GSM one and yours is a TDMA phone, the change only affects GSM. If they are both GSM, it's likely that yours was able to pick up a stronger signal from ATT (you might have a different phone that has better reception in general).

I believe the SIM card in GSM phones identifies the home system of the subscriber. The phone then will always try to connect to that network as the preferred network. If the signal is real bad, it will try to roam onto a stronger network.

Sometimes your phone settings can override this, but even with my Sony Ericsson T68i, it would hop back to AT&T even if I "manually" locked it onto Cingular.