Well, recently he figured out how to take the covers off.... I first tried wrapping tape around the covers, which he pulled off after a few days. So, I Super-glued them on! It worked, he cant get them off now!! Of course when we move from here or I decided I dont need them on anymore I will have to break them to get them off... I dont know how hard it will be to get them off... But its only plastic and there cheep so I can always buy more if needed.
I put the glue in the seems where the two halves of the cover fit together so that I didn't get any glue on the doorknob and didn't damage the door.