I am not a specialist, but with 40 points between PSI and VCI a FSIQ is considered to be pretty much meaningless.

And while the gap between VCI and PRI is < 23 points I seem to remember something about spread between subtests making indexes invalid that make me wonder at the validity of your son's PRI, and maybe even his PSI.

Google WISC-IV Technical Report #4: General Ability Index.
Per that document I think your son's GAI would be 139.