I'm hoping to be in Chicago for a couple of nights in August. I'll be travelling with my 13 year old son. It'd be nice to take him to hear some blues - but I don't know what the laws are concerning under age people in bars? Any advice?

You might want to try checking out
http://www.buddyguys.com/
I think they have all ages shows there, not sure though. Might be a good place to start.
-OR-
If you can switch your travel plans, try coming June 9th - June 12th for Blues Fest in Grant Park.

One more thing, we lives in the burbs so we usually spend the weekend down there. The past few years we have used Priceline.com to get a room down there. We usually end up with the Hilton Palmer House for $60 per night.

The House of Blues Hotel is funky and fun and right next door is the the HOB concert venue and the awesome Sunday Gospel Brunch. The concert schedule for August is not up yet, but here is the website:
http://www.hob.com/venues/clubvenues/chicago/

Thank you both so much for this - my son is really looking forward to visiting Chicago.

On Saturday nights, the Blue Chicago Store hosts a all-ages live blues show in a smoke and alcohol free environment. Families are welcome to enjoy the sounds of the Gloria Shannon Blues Band every Saturday night from 8 p.m. to 12 midnight in a small, intimate setting. There is a $5 cover charge for adults. Children under 12 are free. The Blue Chicago Store is located at 534 N. Clark.

What an absolutely great concept, Rocky! I, for one, would love to see this really catch on. Thanks for bringing it to our attention.

Jazz Showcase has family concerts on Sunday afternoons. They host some of the best jazz in Chicago. It's in the same area as the Clark Street Blues Chicago.