My son got the jumper cables crossed up when attempting to jump start his dead battery. The battery is fine but now there is no power except to his horn and emergency flashers. I'm thinking there is a main fuse blown, but where? When the key is turned on nothing works or lights up. Thanks.

Check all under hood and under dash fuses with a test light or multimeter. It's common in these situations for the ECM and alternator to be damaged.