Here is a step-by-step walk through:
1. Look for the Settings icon and open it
2. Select General > Restrictions
3. Enter your current restrictions passcode
4. At the top of the screen, you will see a "Disable Restrictions" button. Touch this button to disable Restrictions. You will be asked to enter your passcode to do so.
5. Now, the button at the top should have changed to "Enable Restrictions". Touch this button to enable Restrictions anew. You will be asked to enter a passcode. Make sure to enter a new passcode that is not disclosed to your son.
That is it! You have successfully changed your restrictions passcode.