Hi rpervez, welcome to LTH.
Here are a couple old threads about eating out with young kids (not just at nice places, but the higher end places are discussed too):
http://www.lthforum.com/bb/viewtopic.php?t=2956
http://www.lthforum.com/bb/viewtopic.php?t=6688
For your upcoming anniversary, let me just pass along some general advice (much of this is also found in the linked threads). I would recommend either going out for lunch instead of dinner, or going for an early dinner. A place with outdoor seating might be a good choice.
Your son is going to be able to sit still for only a limited amount of time, so whatever you can do to minimize the total time at the restaurant will help (e.g. look at the website ahead of time and think about what you might want to order). Find a place that will have something your son will eat and order it as soon as you are seated, so his food will come out right away. Have something to keep him occupied once he's done eating, a toy he can play with quietly, or paper and crayons.
I will also add one specific suggestion for you to think about: Sabatino's.
http://www.sabatinoschicago.com/
It's not downtown, but I think it would be a nice combination for you of a special-feeling place that is not unwelcoming for kids. We went to Sabatino's when our son was five and he was amazed by the waiters preparing flaming dishes at nearby tables and the sparklers in birthday cakes that we saw going by. He had an order of "tortellini bolognese" which he enjoyed (of course, much of it was brought home in a doggy bag).
Good luck, and happy anniversary. Let us know how it works out.
Amata