My 12-Year-Old Son Has Been Questioning God's Existence. Do You Know of Any Resources that Would Be Helpful to Share with Him? How About for Adults as Well?

I would recommend:

Children Demand a Verdict by Josh McDowell

Answers to Your Kid's Questions by Chuck Colson

Also, for adults:

Case for Faith by Lee Strobel

How Now Shall We Live? By Chuck Colson

Answers for Athiest, Agnostics & Other Thoughtful Skeptics by Calvin Beisner (this is out of print but can probably be purchased through http://www.addall.com/)

Randy Alcorn, founder of EPM

Randy Alcorn (@randyalcorn) is the author of over fifty books and the founder and director of Eternal Perspective Ministries