I forgot my son’s passcode to get into the parental controls. How do I reset it?

Is it the passcode or the parental code that you forgot? Regardless, the process is pretty much to require you to restore the iPad:

• For the iPad's passcode: If you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled - Apple Support
• For Parental Controls: How to Reset Forgotten Restrictions Password on iPhone, iPad - OSXDaily

Again, if this is the Restrictions/Parental Controls passcode, and you don't know it, this iPad must be restored in order to gain access to it. The restore process does NOT require a passcode. However, it does require that you power-off the iPad, and then, connect it directly to a computer's USB port so that iTunes can access it.

If you are still having to enter a passcode, then there may be something else amiss with the iPad and you should make a Genius Bar appointment at your local Apple Store or Apple Authorized Service Provider to have them take a look and provide you with your options.