1. I assume he wasn't on any medication for either test?
2. How focused was he the second time?
3. Was it just one area that pulled down the FSIQ (like processing speed), or multiple areas?
4. Same tester or different tester? I get the impression that a poor tester could try to race through the test or not be as skilled at establishing rapport or getting answers out of the child. I think that could make a big difference.