Do you already have their full reports? I think it would be very telling to look at specific subtests and indexes and see where the changes were. Can you ask the tester to calculate the GAI? That is removing the processing speed from the FSIQ. Maybe that is where the discrepancy is coming from?
If tester can't give it to you and you have the total scaled scores for verbal and performance, then you can find it online.
Yes, you could not retest now, it would not provide reliable scores. Most people advice waiting at least a year between same test, and maybe a bit more for gifted kids. Potentially, you COULD use an alternate test (perhaps SB5). Maybe, instead, you could look into achievement testing for both of them (for younger one, you could then perhaps apply to Davidsons). For older one, achievement being in line with the older or newer test would give you some extra clues.
I really think, without looking at where the discrepancies are, it is hard to make any conclusions.
Another thought; if he does have a LD; then maybe the effect would have amplified as he got older...
