S.S.
We live about 15 minutes from downtown. If he loves trains, you can take him to Navy Pier to ride on the Thomas Train. Millenium Park is awesome for kids of any age. The fountain is great! Depending on when you plan on taking him, check the millenium park schedule online. Wiggle worms will be having free music classes daily all summer long...we plan on spending alot of time there!
As for eating, there is the outdoor cafe at Millenium Park that looks pretty kid friendly. you could also pack sandwiches and eat right next to "The Bean". There are picnic tables with umbrellas.
Have a great day with your little guy!