J.S.
Here's some stuff I do with my boys that are relatively inexpensive:
* Irvine park has a small zoo for $2 per person admission and kids under 1 are free. They also have a train ride for $3 per person and I think kids under 1 are free, too.
http://www.irvineparkrr.com/
*Hulaballoos clubhouse has open playdays in the mornings. Free for adults, and kids under 1 are free if there is a paying child ($6). There is a toddler area that your son would love!
http://hullabaloosclubhouse.com/events.html
* The Pacific Marine Mammal Center rescues seals and sea lions. They give tours and they are FREE!
http://www.pacificmmc.org/
*The La Habra Children's Museum is a hands on kids museum where they can touch everything. Admission is $7 for adults and kids under one are free.
http://www.lahabracity.com/section.cfm?id=41
*The Santa Ana Zoo is a cute small zoo with a playground and a train ride (for $2, I think). Admission is $6 and kids under one are free.
http://www.santaanazoo.org/visinfo.htm
I hope you and your son have some good quality time together!