If you do not know your password ***** install your programs, then you either will have to ask your Son what he set it as.
Or do the below procedure to reset it
Power off the Mac
Right after you hear the chime, press and hold the Command and the R keys for a few seconds, then release. Soon, you'll see the Apple logo and spinning gear (or progress bar).
This will force the Mac to boot into OS X on the Recovery Partition that was created when you installed
When booting is complete, you'll see a window of "OS X Utilities." You won't do anything there. Instead, you'll go to the Menu bar at the top and select Utilities > Terminal.
In the terminal window type the following and hit Return.
A new window will open, like this:
Click on the the main volume, then select the desired user account name in the popup. Then enter the new password (twice). A password ***** is optional. Click "Save."
Go back to the Apple Menu at the top. Select OS X Utilities > Quit OS X Utilities. That will prompt you to restart the Mac.
When the Mac restarts normally, you'll be able to log in to the administrator account with the new password ***** defined above. From there you can manage the rest of of the users if necessary with System Preferences > Users & Groups.