You should have no trouble just getting another sim card from cingular. You may have to bring the 'new' phone to them to get it all programmed. Since you are only changing your equipment without cingular providing anything but a sim card, there should not be any need to modify your existing contract. If you wanted to purchase a new phone from them, with all the rebates and hidden stuff, then you might have to do a contract change. The corporate-owned stores seem to have greater flexibility beyond that of the 'agent' stores.

I strongly suggest you call them and have it disabled for the time being. If it was lost/stolen, a whole lot of airtime can be burned up, which could be your responsibility. After a whole lot of rigamarole, you may get the charges, if any, reversed, but why subject yourself to that hassle? Treat this like a lost/stolen credit card. If your son finds his phone, it would be a simple task to change back to it.