I regularly go to the County Court House in downtown Chicago and have to check whatever blade I am carrying.
The deputy at the gear-check has on several occassions chastised me for having a large Classic Sebenza which has a blade of 3.5 inches, which is half an inch over the legal limit.
This has been confirmed by several LEO in Chicago, both city and county.
FWIW, the knife I carry with me on courthouse days is a Native with a 440V blade, and the cute little deputy behind the desk offers me no grief about this whatsoever.
If your brother is not a dirtball, he's FINE. Chicago cops are, as a rule, very down-to-earth folks and have a LOT more shit on their plate than your brother's pocketknife.
IL law refers to Daggers, Dirks, Bowies, Switchblades or other Automatic Knives, Gravity Knife or any other Dangerous Knife. Dangerous knife, last time I read the code, is not mentioned by length. I don't know where Levine got his text, unless it's been updated recently. I had a friend who was an assistant State's Atty and he read me the code over the phone, told me to stay away from Bowies, daggers and automatics. He also found nothing about blade length.
In other words, it's a HUNTING knife, not a Bowie, don't carry a pointy double-edged blade, and if you insert it in someone, shut up and have your lawyer do the talking.