If you don't mind getting rid of them, you can go to your library and remove the books from your Kobo account. You can do this either on the ereader by long pressing a title and selecting remove from account, or on the computer desktop with a right click. When the e-reader syncs again the title will be gone. I don't think you can get them back again though!
Alternatively, do a factory reset (under settings/device information) and set up a separate account for your son's device. You will not be able to get any titles you bought for him to the device though.
That will keep him safe for about 3 to 4 years, when he figures out how to get whatever books he want on his own.
