On a different angle from what previous posters are mentioning.....

When your son arrived at the hospital, he is going to be asked: is this a work related accident?  Who pays will be determined by whether it was a work related accident.  Now, is it routine for you to call your son to help move equipment? I can see where this is part of your farm operation practices, and this is a work-related accident.

This might be an issue you need to have a discussion with your insurance agent about: if I have someone help move equipment occassionally, is that person covered if there is an accident?  Something to think about.