You may be able to confirm of deny, but my (American) brother-in-law reckoned his accent in Roger Rabbit was 100% spot-on Chicago.
To me, it sounded like an Englishman doing a generic American gangster/gumshoe accent, which tend to be more New York-ish. It's def not a spot on Chicago accent. I think that was his first role playing an American, and he did it brilliantly. I have an ear for accents, so I could tell he was English. But most people couldn't. And in his subsequent roles playing Americans, his accent got much better.
In fairness to your B-I-L, sometimes the a Chicago accent can sound like a NY accent. For a spot on Chicago accent, watch anything the late Dennis Farina was in.