Hm, odd, the hasbro ones are supposed to be reasonably sturdy.
Well take the time to teach him how to use it properly, I guess, on the condition if he permanently breaks another one he won't get any more chances, or something. That said if you've tried that already, get him a light launcher LR and talk to your parents about preventing him from using your string launcher because that many breaks is not normal.
I use household oil/machine oil. I'm not really sure what is usually used, though, or whether organic oils are useable or not.