Do you think that this is fair? Is there anything I can do?

No it is not fair and he should talk to his dad about his education and let him know how important it is to him.He is working his son to hard and should be concerned about the problems he is having.Such a young age to have problems with his back.He needs to speak up for himself.Let him know to tell his dad how much he likes the work but does not want to jeopardize his health to being disabled.