A 42" child will be able to go on many, but not all, of the rides at WDW. If you go to the Little Ones planning section of the WDW website, found
HERE, you'll see on the right a link to a downloadable PDF called Attraction Height Requirements. Any ride NOT listed on this PDF has no height requirement and may be enjoyed by anyone.
Your son can also go on the following rides for which there is a height requirement lower than 42":
The Barnstormer at the Magic Kingdom
Splash Mountain at the MK
Big Thunder Mountain at the MK
Stitch's Great Escape at the MK
Test Track at Epcot
Soarin' at Epcot
Tower of Terror at Disney's Hollywood Studios
Star Tours at DHS
Dinosaur at the Animal Kingdom
Kali River Rapids at the AK
Overall, out of the MANY dozens of activities at WDW, I see only five rides in all of the Disney theme parks that your son would be too short to ride. This would definitely not be enough to keep me away from WDW.
Good luck with your planning!