I also get a GAI of 150 using the using the WISCIV Technical Report #4 and your son's subscale scores.
Our tester cited Flanagan and Kaufman (2004) "Essentials of WISC-IV Assessment" stating that the GAI may be calculated as a measure of general intellectual ability if the difference between composite scores is 23 points or more, as long as the difference between VCI and PRI is less than 23 points. Your sons scores meet those criteria. Others use slightly different criteria, but I don't think anyone says it "must" be calculated and thus not every tester will do it. It seems it would be worth asking your tester if he/she thinks it would be appropriate to add it to the report.
