There are many restaurants in the Chicago area that offer gift cards/certificates. However, I recommend getting them for one of the several
groups of excellent restaurants in town, so that the recipient has a choice among at least several different restaurants with several kinds of food at various price points. In addition to the flexibility in choosing a restaurant, this also gives them the ability to go to a different place if they don't use the entire balance, rather than being forced to choose between going back to a place they're already been, or discarding the remaining balance.
These groups and gift card/certificates include:
1. Lettuce Entertain You (
http://www.leye.com ) is a group of several dozen restaurants, mostly in the Chicago area. Its offerings include high-end creative places (Tru, Everest, and L2O), as well as mid-priced and other places. L2O, a high-end seafood restaurant downtown, is their newest restaurant. Reel Club is another new restaurant of theirs, also specializing in seafood but mid-priced; I've been there and it's excellent. Also, Reel Club is in Oak Brook, very convenient to Oak Park, so this would be the perfect place for your brother. The gift certificates, which you can buy on their website, are good at any of their restaurants. Between Nov 1 and the end of the year, the Lettuce Entertain You group offers a free, lightly-restricted $25 bonus certificate for each $100 of gift cards purchased. So you can get "more bang for the buck" this way.
2. Levy (
http://www.levyrestaurants.com ) is another group of restaurants in the Chicago area. They only have a few restaurants in the Chicago area, but they are excellent: Spiaggia, an extremely high-end Italian restaurant; Cafe Spiaggia, its mid-priced Italian sibling next door; Bistro 110, a French bistro; and Jake Melnick's Corner Tap. Between Thanksgiving and the end of the year each year, Levy offers a similar deal with a free, lightly-restricted $25 bonus certificate for each $100 of gift cards purchased.
3. KDK Restaurants (
http://www.kdkrestaurants.com ) has four restaurants: Red Light (Asian fusion), Gioco (Italian), Opera (Chinese), and Marche (French bistro). Between Thanksgiving and the end of the year each year, the KDK group gives a similar, but $50, bonus certificate with each $100 purchased.
4. American Express (
http://www.americanexpress.com ) offers gift cards that are good at any restaurant that accepts their credit card. Which gives them a huge array of options, letting them decide what place most appeals to them.