Here is some things you can try:
Use a spyware removal tool. like this one: The Cleaner 2011 - Download
(don't know if this software is good or not. i haven't tried it and don't know if it will help)
you could try to look at the resent installations in the "uninstall a program" section in control panel.
just sort by latest installs. if you see any odd software you don't remember installing sense your son started stealing your codes.
I think most keyloggers don't show up as a "installed" software, in that case you will need to find it and remove manually.
You might be able to get the name on the software by checking the download history in your browser.
He might have downloaded the software from your computer instead of transferring it with a USB device.
If you find the name of the keylogger you can google how to remove it.
you might also find the keyloggers name by looking at the task manager under the tap processes. problem is that you properly don't know what 10% of it is anyway. so this is gonna be difficult for you. sort that list by "cpu" so you can see where the processor is using performance.
Hit that kid with a belt until he tells how he obtained the codes.