To answer your question, the Firearm Owner Id card just allows him to have a firearm in IL, buy one in the state, and buy ammo, that's it. Being from WI, it's what someone is allowed to do, but not have a card to do it.

As far as you giving him a firearm, you didn't bring up anything specific, but for the most part your fine unless he lives in Chicago or some of the suburbs close to the city, where the local laws get kind of strange in terms of what they are, and how they are enforced. As long as it's not an assault rifle or something like one that you hear about in the news you shouldn't have a problem.

Now if you're giving him a braced AR pistol and he lives in IL, along what has been proposed and will probably go into place after the elections, that's a different problem that you can start a new post about