J.L.
First of all, Welcome to Sactown!...you will love living in Midtown...great place, people and of course food!. They just openend up a great kids place called ArtBeast Studio... http://www.artbeaststudio.com/
I have not been there yet, but we plan to get out there this summer since we are over by Mather Airport.
I am not sure about the Parks and Rec out in that area..but you might want to check out meetup.com and look for mommy playgroups in that area..I think there is one for the midtown area. When I first moved here 3years ago, I didn't know a soul, so I started my own mommy group and is still going strong.
oh, another place your son might like is the California Train Musuem...they have a Thomas the train play area upstairs..and the membership there is really cheap....a kids membership is $20 I think and a parent gets in free with the child...this place is in old sac.
we used to take Kindermusic in midtown, wonderful lady that teaches there...she's like off of J and 24th street I think...
Hopefully this will get you going....and if you can't find a mommy group there...start one! Best thing I ever did!