IST is a really modern and easy airport, especially when transferring TK to TK. He will get off the first leg and can hang out in the terminal. As a transfer, he will not go through immigration and will just go from one gate to the other. He probably will not get his IST-ICN boarding pass at check in in ORD (given its the next day). So, you will need to make sure he knows to go to the "Transfer Desk" to get his boarding pass. Have him ask about this when he checks in at ORD.

Also, like most airports (other than the US) he should not go to the actual gate until it says so on the board. There is nothing at the gate, except seats. And typically once you enter, your stuck there. So, tell him to watch the display board to direct him to go to the gate. It will probably open 30 minutes or so before boarding. I say this, because it may be different from what he is accustom to flying in the US.

There is a Starbucks in the terminal, so that should make him feel at home.

No reason to worry or stress. If he has any questions, just make sure he knows to ask any TK staff or at the info counter. Most people working in the airport can speak English or help him find someone who can help. Its an easy airport.

Cheers