I wonder if you can't get the actual test details by writing a letter requesting your ds' school records? It would really help to know the name of the test, and whether or not it was an ability or an achievement test etc. to make some sense out of the results.
Re you question is your ds gifted - absolutely! He's had private testing and testing at Northwestern that both show he's gifted, so don't for one minute doubt he's gifted. There is so much that could have happened with the group testing through school - everything from having a bad day to simply being one of those kids who doesn't test well on one particular type of test.
If you want him in the program that he was testing for, I'd make an appeal in writing. Send copies of his previous testing with it and request he be considered for the program. If you can find out the name of the test he was given first, and gather some information about it that might help you understand the results, that would help too. You might also try to find out from another parent or teacher (if you can) what ability test is used in the school district's SPED department for IEP eligibility and ask that he be tested with it individually. Usually (in the school districts I'm familiar with) either the WISC or the WJ-III Test of Cognitive Abilities is used, and unless your ds has a really negative experience in some way, he's bound to test gifted using a true ability test based on his past testing. Anyway, I'm not in Chicago, so I can't help you with what will happen there, but we did successfully advocate for our ds in our school district when his CogAT score came out lower than his previous private WISC. It just took a bit of advocating and not taking "no" for an answer, and also having a kid who had qualifying scores.... sometimes you may find that school district personnel are really quick to be negative about a parent questioning a test result or being the squeaky wheel, but sometimes that's because they are used to hearing from squeaky parents who don't have kids with truly qualifying abilities... which isn't your kid - you've got the data you need to back up your advocacy

Best wishes,
polarbear