What ever Official ID that he would need if he boards with you ....
Actually, YOU might need his passport / birth certificate. Others will know better.
He WILL need his DL to get into the Port Area.
He will have to check in to get his KTTW card and get his security picture taken before boarding.
I think that when we did this in the past, DW had my Passport and checked in while I had my DL to return to the port area.